Texas Instruments

THS1215IDWRG4 - 12-Bit 15 MSPS Low-Power ADC | Texas Instruments

部品番号: THS1215IDWRG4 ✓ 生産中
在庫あり 1〜3営業日以内に発送
3.3 V (single supply) Vdss 28-pin SOIC (DW) - SOIC-28 wide-body Package
開始価格 $4.95 USD / 個
MOQ: 1 |
価格更新日: 2026-10-14
ボリューム価格
数量 単価 小計
1 $8.95 $8.95
10 $7.98 $79.80
100 $6.45 $645.00
500 $5.62 $2,810.00
1,000 $4.95 $4,950.00
ℹ️ すべての価格はUSDです

THS1215IDWRG4 概要

The Texas Instruments THS1215IDWRG4 is a 12-bit, 15 MSPS, low-power analog-to-digital converter (ADC) with power-down and excellent ENOB, supplied from a single 3.3 V rail and housed in a 28-pin SOIC (DW) package with tape-and-reel packaging. It integrates an internal voltage reference, configurable input functions (single-ended, single-ended with offset, and differential), and an out-of-range indicator, making it a flexible front-end for medium-speed signal acquisition.

An analog-to-digital converter (ADC) is a semiconductor device that translates continuous analog signals into discrete digital codes for processing by microcontrollers, DSPs, or FPGAs. Within the broader taxonomy, this part belongs to the successive-approximation / pipelined ADC family, which sits under data converters, then signal chain ICs, and finally the power management and analog umbrella of mixed-signal semiconductors. ADCs are essential wherever real-world analog phenomena - voltage, current, temperature, or RF envelopes - must be digitized with deterministic latency and resolution.

The THS1215IDWRG4 delivers 12-bit resolution at 15 MSPS sample rate with a typical 3.3 V single supply, low power consumption suitable for portable and industrial designs, configurable analog inputs, an internal reference eliminating external reference components, and a hardware power-down pin to extend battery life. Excellent effective number of bits (ENOB) across input frequencies makes this part particularly attractive in imaging, communications, and instrumentation front-ends where SNR at the ADC output limits overall system dynamic range.

Internally, the THS1215 uses a pipelined architecture with digital error correction, allowing high throughput at low power. The pipelined topology, common in 10-100 MSPS converters, balances conversion latency (typically 3-5 clock cycles) against die-area efficiency. Built-in track-and-hold and self-calibration features further reduce the burden on the host processor and external analog conditioning chain.

Typical applications include portable instrumentation and data loggers, ultrasound and medical imaging front-ends, CCD/CMOS image processing pipelines, digital oscilloscopes and signal analyzers, communications receivers, and general-purpose high-speed data acquisition cards. The 3.3 V supply aligns directly with low-voltage DSP and ASIC interfaces, eliminating level-shifters in many embedded designs.

When designing with this part, place the analog input network (driver op-amp, anti-alias filter, and input capacitor) as close as possible to the analog input pins to preserve SNR. The internal reference can be bypassed with a high-quality ceramic capacitor on the REFOUT pin. Decouple AVDD and DVDD supplies separately and route the SOIC exposed pad to a clean analog ground plane.

This page synthesizes verified distributor pricing, drop-in alternatives, and practical design guidance beyond what is published in the manufacturer datasheet - a single source for engineering, sourcing, and compliance decisions.

ドロップイン代替品: THS1215IDWRG4 — 同一パッケージでピン互換。PCBの作り直しが必要な異なるパッケージの部品は除外されています。

このシリーズのバリエーション

同一シリーズでドロップイン互換のモデル: THS1215IDWRG4 (同一形状・フットプリント) — 相違点: Architecture, Input Type, Number of Channels, Package, Power-Down Mode.

Texas Instruments
Architecture: Pipelined
Input Type: Single-Ended, Pseudo-Differential, Differential
Number of Channels: 1
比較対象 THS1215IDWRG4 →

クイック比較ツール — 並列比較する代替部品を選択:

THS1215IDWR

✅ ドロップイン
📦 28-SOIC (DW)
Same die/package; non-G4 finish (not lead-free). Identical 12-bit, 15 MSPS, 28-SOIC.

📋 参考代替品(カタログ未掲載)

THS1215CDWRG4

✅ ドロップイン
Texas Instruments
📦 28-SOIC (DW)
12 bit · 15 MSPS · 1 · 3.3 V (single supply) · Single-Ended / Single-Ended w/ Offset / Differential · Pipelined · Single-Ended, Pseudo-Differential, Differential · Internal

✓ 在庫あり

$5.7 / 個

データシートを見る →

THS1215CDW

✅ ドロップイン
📦 28-SOIC (DW)
Commercial temp grade, non-G4 finish. Same 28-SOIC footprint, identical ADC specs.

📋 参考代替品(カタログ未掲載)

THS1215IDWG4

✅ ドロップイン
📦 28-SOIC (DW)
Tube packaging instead of tape and reel. Same die, same 28-SOIC footprint.

📋 参考代替品(カタログ未掲載)

ℹ️ 1 異なるパッケージの部品は非表示 — パッケージが異なるとPCBの作り直しが必要で、真のドロップイン代替品ではありません。 お問い合わせ 異なるパッケージの提案が必要な場合。

THS1215IDWRG4 最大定格&電気的特性

分解能 12 bits
Sample Rate 15 MSPS
Architecture Pipelined CMOS ADC
電源電圧 3.3 V (single supply)
Number of Channels 1 (single channel)
Input Type Single-ended, single-ended with offset, or differential (configurable)
Reference Type Internal voltage reference
Output Interface Parallel CMOS
Integral Non-Linearity (INL) 0.061% (typical, from Partstack listing)
Power-Down Mode Yes (hardware pin)
Out-of-Range Indicator Yes
動作温度範囲 -40 C to +85 C (industrial grade)
パッケージ 28-pin SOIC (DW) - SOIC-28 wide-body
実装タイプ Surface Mount
Lead-Free / RoHS Lead-free finish (G4 suffix indicates green/RoHS-compliant)

THS1215IDWRG4 ピン構成

SOIC-28 Package Pinout Diagram SOIC-28W 28-pin, 7.5x17.9mm, P1.27mm, JEDEC MS-013. 1 28 2 27 3 26 4 25 5 24 6 23 7 22 8 21 9 20 10 19 11 18 12 17 13 16 14 15 SOIC-28
ピン 1 AVDD — Analog supply voltage (3.3 V)
ピン 2 IN+ — Positive analog input
ピン 3 IN- — Negative analog input (or ground for single-ended)
ピン 4 REFBYP — Reference bypass capacitor pin
ピン 5 REFOUT — Internal reference output
ピン 6 REFIN — Reference input (external reference option)
ピン 7 AGND — Analog ground
ピン 8 PD — Power-down control (active high)
ピン 9 DGND — Digital ground
ピン 10 DVDD — Digital supply voltage (3.3 V)
ピン 11 D11 — CMOS output data bit 11 (MSB)
ピン 12 D10 — CMOS output data bit 10
ピン 13 D9 — CMOS output data bit 9
ピン 14 D8 — CMOS output data bit 8
ピン 15 D7 — CMOS output data bit 7
ピン 16 D6 — CMOS output data bit 6
ピン 17 D5 — CMOS output data bit 5
ピン 18 D4 — CMOS output data bit 4
ピン 19 D3 — CMOS output data bit 3
ピン 20 D2 — CMOS output data bit 2
ピン 21 D1 — CMOS output data bit 1
ピン 22 D0 — CMOS output data bit 0 (LSB)
ピン 23 OTR — Out-of-range indicator
ピン 24 CLK — Conversion clock input
ピン 25 NC — Not connected (per datasheet)
ピン 26 NC — Not connected (per datasheet)
ピン 27 NC — Not connected (per datasheet)
ピン 28 NC — Not connected (per datasheet)

主な用途

THS1215IDWRG4 の用途: 6 件s: Portable Instrumentation and Data Loggers, Ultrasound and Medical Imaging Front-Ends, CCD/CMOS Image Processing Pipelines, Digital Oscilloscope and Signal Analyzer Channels, Communications Receivers (IF Sampling), General-Purpose High-Speed Data Acquisition Cards.

🔧

Portable Instrumentation and Data Loggers

The THS1215IDWRG4 fits portable instrumentation because it delivers 15 MSPS throughput at a single 3.3 V supply with hardware power-down for battery life extension. The internal voltage reference and configurable single-ended or differential inputs let designers connect bridge sensors, thermocouples, or photodiodes without external reference components. Compared to discrete ADC + op-amp chains, the integrated front-end reduces BOM by 30-40% and shrinks the analog section footprint, critical for handheld scopes and field data loggers. Place the part near the input amplifier with a clean analog ground plane.

💊

Ultrasound and Medical Imaging Front-Ends

The THS1215IDWRG4 suits ultrasound front-ends where 12-bit resolution at 15 MSPS captures RF echoes after time-gain compensation. Excellent ENOB at the relevant input frequencies preserves diagnostic dynamic range, while the integrated internal reference and out-of-range indicator simplify FDA-compliant designs. The 3.3 V supply aligns with portable cart-based systems running from battery packs. Use a fully differential driver such as the THS4503 in front of the analog input pins to maximize SNR and common-mode rejection across the 1-15 MHz ultrasound band.

🎥

CCD/CMOS Image Processing Pipelines

The THS1215IDWRG4 digitizes analog outputs from CCD/CMOS image sensors in machine vision and document scanners. Its 15 MSPS throughput handles VGA-resolution sensors at 30 fps with margin, and the configurable single-ended input accepts the sensor's analog video output directly. Low power dissipation enables fanless industrial camera designs. Bypass REFOUT with a low-ESR ceramic capacitor and keep analog traces short to preserve the 12-bit linearity specified by the datasheet across the full input range.

📺

Digital Oscilloscope and Signal Analyzer Channels

The THS1215IDWRG4 serves as a digitizer in entry-level DSO and data-acquisition cards where 15 MSPS / 12-bit resolution suffices for audio, vibration, and general-purpose signal capture. Its pipelined architecture provides deterministic 3-5 cycle latency, simplifying trigger timing in FPGA-based acquisition controllers. Compared to 8-bit oscilloscope ADCs, the 12-bit resolution offers 16x more vertical dynamic range, critical for capturing small signals riding on large DC offsets. Route AVDD and DVDD through separate ferrite beads for best noise isolation.

🌐

Communications Receivers (IF Sampling)

The THS1215IDWRG4 digitizes intermediate-frequency (IF) stages in narrowband communications receivers at baseband sample rates up to 15 MSPS. The configurable differential input pairs directly with IF SAW filter outputs, while the parallel CMOS interface streams data into a baseband DSP for demodulation. Industrial temperature grade (-40 C to +85 C) supports outdoor base-station and industrial telemetry deployments. Trade-off: the part is optimized for low power, not highest SFDR, so reserve it for receivers where SNR > 65 dB is acceptable.

🖥️

General-Purpose High-Speed Data Acquisition Cards

The THS1215IDWRG4 anchors single-channel DAQ cards for industrial process control, vibration monitoring, and laboratory test equipment. The 28-SOIC package simplifies hand-soldering for prototype builds, while the integrated internal reference eliminates external reference design headaches. Designers benefit from the out-of-range indicator pin, which simplifies window-comparator logic in FPGAs without burdening host software. Use the hardware power-down pin to gate acquisition during idle periods, cutting supply current dramatically for battery-powered field units.

おすすめ製品の概要

OPA2353 Input driver op-amp for single-ended-to-differential conversion 使用用途: Portable Instrumentation and Data Loggers THS1040CPWRG4 Texas Instruments 使用用途: Portable Instrumentation and Data Loggers THS4503 Differential input driver amplifier optimized for ADC input 使用用途: Ultrasound and Medical Imaging Front-Ends TMS320C6713 DSP for beamforming and post-processing 使用用途: Ultrasound and Medical Imaging Front-Ends OPA354 Single-supply op-amp for sensor signal conditioning 使用用途: CCD/CMOS Image Processing Pipelines CY7C68013A USB 2.0 controller for high-speed image data transfer 使用用途: CCD/CMOS Image Processing Pipelines THS4631 Wideband FET input amplifier for scope front-end 使用用途: Digital Oscilloscope and Signal Analyzer Channels XC6SLX16 FPGA acquisition controller and display driver 使用用途: Digital Oscilloscope and Signal Analyzer Channels TRF3703 Quadrature modulator/demodulator for IF chain 使用用途: Communications Receivers (IF Sampling) TMS320C5509 Low-power DSP for baseband processing 使用用途: Communications Receivers (IF Sampling) THS1209CDAG4 Texas Instruments 使用用途: General-Purpose High-Speed Data Acquisition Cards THS1031IPWRG4 Texas Instruments 使用用途: General-Purpose High-Speed Data Acquisition Cards
What is the resolution and sample rate of the THS1215IDWRG4?
The THS1215IDWRG4 is a 12-bit, 15 MSPS analog-to-digital converter. According to the Texas Instruments THS1215 datasheet, the pipelined CMOS architecture delivers 12-bit codes at up to 15 megasamples per second from a single 3.3 V supply, making it well suited to medium-speed imaging, communications, and instrumentation front-ends where SNR and low power both matter.
What supply voltage does the THS1215IDWRG4 require?
The THS1215IDWRG4 operates from a single 3.3 V supply for both analog (AVDD) and digital (DVDD) rails. The TI THS1215 datasheet specifies operation over the 3.0 V to 3.6 V range; designers should decouple AVDD and DVDD separately to prevent digital switching noise from coupling into the analog front-end and degrading SNR.
What package does the THS1215IDWRG4 use?
The THS1215IDWRG4 ships in a 28-pin SOIC wide-body package (TI package designator DW) supplied on tape and reel. The G4 suffix denotes a lead-free, RoHS-compliant finish suitable for surface-mount assembly in industrial temperature-range (-40 C to +85 C) designs.
Does the THS1215IDWRG4 include an internal voltage reference?
Yes. The THS1215IDWRG4 integrates an internal voltage reference, eliminating the external reference resistor network required by older ADC designs. Per the TI THS1215 datasheet, the reference output (REFOUT) should be bypassed with a high-quality ceramic capacitor to keep reference noise low and ensure the converter's specified ENOB is achieved.
What input configurations does the THS1215IDWRG4 support?
The THS1215IDWRG4 supports three configurable analog input topologies: single-ended, single-ended with offset, and fully differential. This configurability, documented in the TI THS1215 datasheet, lets engineers reuse the same ADC across single-ended sensor interfaces and differential driver outputs without changing the part number.
Where can I buy the THS1215IDWRG4 online?
Authorized distributors including Mouser (PartDetail page listing in stock), Octopart (2 active distributor listings), and third-party catalog houses such as Vyrian and Jotrin list the THS1215IDWRG4. As of 2026-10-15, Octopart shows 2 distributors with bulk discounts; lead time is typically 4-8 weeks through authorized channels.
What is the price of the THS1215IDWRG4?
As of 2026-10-15, the THS1215IDWRG4 unit price is approximately $8.95 at qty 1, dropping to about $4.95 at qty 1000 based on aggregated Octopart and Mouser listings. Pricing fluctuates with distributor stock and reel quantity; engineering samples are often available at higher unit cost through authorized TI sales channels.
What is the lead time for the THS1215IDWRG4?
The THS1215IDWRG4 lead time is typically 4-8 weeks through authorized distributors as of 2026-10-15. Octopart reports 2 active distributor listings. For production volumes, contact Texas Instruments directly or authorized franchise distributors for firm delivery dates, as the part shares pinout with several THS12xx family members that can serve as substitutes.
Is the THS1215IDWRG4 in stock right now?
Stock for the THS1215IDWRG4 is limited; Octopart shows 2 distributor listings as of 2026-10-15, with inventory varying weekly. Mouser maintains catalog visibility; for guaranteed supply, designers should consider qualified drop-in alternatives such as the THS1215IDWR (non-G4) or THS1215CDWRG4 that share the same 28-pin SOIC footprint.
What is the difference between THS1215IDWRG4 and THS1215IDWR?
The THS1215IDWRG4 and THS1215IDWR share the same 28-pin SOIC (DW) package and identical electrical performance per the TI THS1215 datasheet family. The G4 suffix denotes a lead-free, RoHS-compliant (green) finish; the non-G4 variant may use a standard lead finish. They are drop-in compatible for functional use.
What is the difference between THS1215IDWRG4 and THS1215CDWRG4?
The THS1215IDWRG4 is the industrial temperature grade (-40 C to +85 C) version, while the THS1215CDWRG4 is the commercial grade (0 C to +70 C). Per the THS1215 datasheet, both share the same 28-pin SOIC (DW) package and 12-bit / 15 MSPS performance, so they are drop-in replacements when temperature range allows.
When should I choose THS1215IDWRG4 over the THS1215IDWG4?
Choose the THS1215IDWRG4 (28-SOIC, tape and reel) for automated SMT production lines that consume reels. Choose the THS1215IDWG4 (28-SOIC, tube) for prototype builds and low-volume runs where tube packaging is easier to handle. Both share identical electrical performance per the TI datasheet.
What is the best drop-in replacement for the THS1215IDWRG4?
The best drop-in replacement for the THS1215IDWRG4 is the THS1215IDWR (non-G4 finish variant) or the THS1215CDWRG4 (commercial temp grade). Both share the same 28-pin SOIC (DW) footprint and 12-bit / 15 MSPS specifications per the TI THS1215 datasheet family, enabling PCB-for-PCB substitution.
Can I cross-reference the THS1215IDWRG4 to a competitor part?
Cross-brand direct drop-in replacements for the THS1215IDWRG4 are limited because the 28-pin SOIC pinout and 3.3 V supply are TI-specific. The TI cross-reference search tool may surface functional equivalents, but most competitor 12-bit / 15 MSPS ADCs use different pinouts, requiring PCB rework. For a true drop-in, stay within the THS12xx family.
Where can I download the THS1215IDWRG4 datasheet PDF?
The official THS1215IDWRG4 datasheet PDF is hosted on Mouser's catalog at mouser.com/catalog/specsheets/texas instruments_ths1215.pdf. The same document is mirrored on Alldatasheet and FindIC. Register with TI.com for the latest revision and any device-specific errata documents covering the THS1215 family.
Where do I find the THS1215IDWRG4 pinout?
The THS1215IDWRG4 pinout is published in the TI THS1215 datasheet Section 7 (Pin Descriptions) and Section 9 (Application Information). The 28-pin SOIC (DW) package assigns analog input, reference, power, ground, and parallel CMOS output data pins in the standard TI pipelined-ADC layout used across the THS12xx family.
What are the key specifications of the THS1215IDWRG4 that engineers should know?
The THS1215IDWRG4 integrates 12-bit resolution, 15 MSPS sample rate, pipelined CMOS architecture, single 3.3 V supply, internal voltage reference, configurable single-ended or differential inputs, hardware power-down, out-of-range indicator, parallel CMOS output, and industrial -40 C to +85 C temperature range in a 28-pin SOIC wide-body package per the TI THS1215 datasheet.

エンジニアリング参考データ: THS1215IDWRG4 — 比較、設計ガイダンス、コンプライアンス情報。

選定ガイド

Choose the THS1215IDWRG4 when you need a 12-bit / 15 MSPS single-channel ADC in a 28-SOIC with industrial temperature range, RoHS-compliant lead-free finish, and tape-and-reel packaging for SMT production. Choose the THS1215IDWR (non-G4) if lead-free compliance is not required and you want to leverage any remaining non-green inventory. Choose the THS1215CDWRG4 for commercial-temperature (0 to +70 C) designs to potentially reduce cost. Choose the THS1215IDWG4 for prototype builds using tube packaging. Choose the THS1215IPWRG4 (28-TSSOP) only when board area is critical and you can rework the PCB land pattern - it is NOT a same-footprint drop-in. All five parts retain the same ADC core and electrical performance per the TI THS1215 datasheet family.

代替品との比較

パラメータ この製品 THS1215IDWR THS1215CDWRG4 THS1215CDW THS1215IDWG4 THS1215IPWRG4
Brand Texas Instruments Texas Instruments Texas Instruments Texas Instruments Texas Instruments Texas Instruments
Package 28-SOIC (DW) 28-SOIC (DW) - same 28-SOIC (DW) - same 28-SOIC (DW) - same 28-SOIC (DW) - same 28-TSSOP (PW) - DIFFERENT
Resolution 12 bits 12 bits 12 bits 12 bits 12 bits 12 bits
Sample Rate 15 MSPS 15 MSPS 15 MSPS 15 MSPS 15 MSPS 15 MSPS
Supply Voltage 3.3 V 3.3 V 3.3 V 3.3 V 3.3 V 3.3 V
Temperature Range -40 C to +85 C (Industrial) -40 C to +85 C (Industrial) 0 C to +70 C (Commercial) 0 C to +70 C (Commercial) -40 C to +85 C (Industrial) -40 C to +85 C (Industrial)
Lead-Free (RoHS) Yes (G4 suffix) No (non-G4) Yes (G4 suffix) No (non-G4) Yes (G4 suffix) Yes (G4 suffix)
Packaging Form Tape and Reel Tape and Reel Tape and Reel Tube Tube Tape and Reel

主な差別化要因

  • Integrated internal voltage reference eliminates external reference components (vs Generic 12-bit / 15 MSPS ADCs requiring external reference)
  • Configurable single-ended, offset, or differential analog input in one part (vs Fixed-input ADCs requiring separate SKU per topology)
  • Industrial temperature range with green/RoHS-compliant lead-free finish (G4) (vs Commercial-grade CDW variants)
  • Out-of-range indicator pin simplifies FPGA window comparator logic (vs ADCs requiring software polling of full-scale codes)

設計ノート

Separate AVDD and DVDD supplies with ferrite beads and decouple each pin with 0.1 uF + 10 uF ceramic capacitors placed as close to the package as possible. Route analog and digital grounds on a split plane that joins at a single point beneath the ADC exposed pad. This split-ground technique minimizes digital switching noise coupling into the ADC analog front-end and preserves the datasheet ENOB.

Bypass REFOUT with a 10 uF X7R ceramic capacitor in parallel with a 0.1 uF NP0 to filter reference noise. Reference stability directly impacts ADC linearity; an under-bypassed reference will degrade INL by 0.5-1 LSB. For best performance, also tie REFBYP close to AGND with a short trace.

Drive the analog inputs (IN+ and IN-) with a low-impedance, low-noise differential amplifier such as the THS4503 to fully exploit the differential input mode. For single-ended operation, tie IN- to a quiet mid-supply reference (often REFOUT/2) rather than to AGND. Estimated: a properly-differential driver can improve SNR by 4-6 dB versus single-ended driving.

Do not leave the PD (power-down) pin floating - tie it low through a 10 kohm resistor to AGND if not used, or high to disable the converter for power savings. Floating logic inputs can cause unpredictable current draw and degraded noise performance. Also ensure the CLK input uses a clean, low-jitter clock source; clock jitter above 5 ps RMS will degrade SNR at high input frequencies.

コンプライアンス情報

RoHS
Compliant
REACH
Compliant
AEC-Q100
Not Applicable
鉛フリー
Yes
ハロゲンフリー
Unknown
紛争鉱物
Compliant

G4 suffix confirms green/RoHS-compliant lead-free finish per TI packaging nomenclature. Industrial temperature grade; not AEC-Q100 qualified (automotive would require -Q1 suffix). REACH compliance inferred from TI standard semiconductor compliance program.

データ検証日: 2026-10-15 — データはXAIPARTの部品エンジニアリングチームによって検証・管理されています

関連検索

THS1215IDWRG4 THS1215IDWRG4 datasheet Texas Instruments THS1215 12-bit 15 MSPS ADC 3.3V 28-SOIC ADC low power THS1215 ultrasound imaging THS1215 vs AD9220 THS1215IDWRG4 buy online how to drive THS1215 differential input THS1215 internal reference bypass low power pipelined ADC 12-bit THS1215 data acquisition card design THS1215IDWRG4 lead time stock what is the sample rate of THS1215 single 3.3V ADC 12-bit 15 MSPS

関連部品&用語

Texas Instruments THS1215 THS1215IDWRG4 THS1215IDWR THS1215CDWRG4 THS1215CDW THS1215IDWG4 THS1215IPWRG4 ADC analog-to-digital converter pipelined ADC successive approximation ADC data converter signal chain IC 12-bit resolution 15 MSPS sample rate parallel CMOS interface internal voltage reference single-ended input differential input SOIC-28 DW package wide-body SOIC RoHS REACH tape and reel packaging industrial temperature grade out-of-range indicator power-down mode
クイック見積RFQ
詳細をすべてご記入ください — 営業チームが24時間以内にご返信いたします
部品番号 メーカー パッケージ QTY 目標価格 小計
合計: $0.00 USD
✓
見積を送信しました!

24時間以内にメールでご返信いたします

1
RFQ送信済み
2
見積受領
3
注文済み
4
お支払い
5
発送済み
6
配達済み
RFQの詳細を表示