MT88L70AS1 - 3V DTMF Receiver SOIC-18 | Microchip Technology
MPN: MT88L70AS1 ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $2.85 | $2.85 |
| 10 | $2.55 | $25.50 |
| 100 | $2.1 | $210.00 |
| 500 | $1.75 | $875.00 |
| 1,000 | $1.45 | $1,450.00 |
MT88L70AS1 Overview
A DTMF receiver is a tone-decoding interface IC used in telephone and VoIP systems, security panels, remote control, and industrial command decoders to detect standard 16-tone DTMF pairs transmitted on the audio path. It belongs to the broader class of telecom interface ICs (hypernymy: DTMF receiver -> tone decoder -> telecom interface IC -> interface IC -> integrated circuit). DTMF receivers remain relevant in legacy PSTN endpoints, two-way radio signalling, and any audio-band command channel where robust in-band signalling is required.
The MT88L70AS1 operates from a 3V supply, making it well suited to battery-powered and portable equipment. Key features include switched-capacitor band-split filters for the standard high-group and low-group DTMF frequencies, a digital counting decoder that outputs a 4-bit code for the 16 DTMF pairs, and an internal gain-setting amplifier that allows direct connection to the line. An adjustable guard time minimises false decoding on transient tones, while power-down and inhibit modes reduce quiescent current in standby.
Architecturally, the device combines analog switched-capacitor filtering with digital decoding, allowing the entire DTMF receiver function with minimal external components. The MT88L70AS1 is the 18-pin SOIC variant of the MT88L70 family (also available in 20-pin SSOP as MT88L70AN1 and in 20-pin SSOP tape-and-reel). The original part was developed by Mitel/Zarlink; Microchip acquired the product line and continues to manufacture it as an active legacy part.
Typical applications include telephone answering machines, remote control decoders, security alarm panels, two-way radio signalling, industrial command-over-audio links, and any system that needs to detect 16-tone DTMF signalling from a 3V supply rail. Designers of new low-power consumer or industrial products that interface to legacy PSTN or audio-band command channels frequently select the MT88L70AS1 for its 3V operation, central-office decoding quality, and proven pin-out.
When designing with this device, ensure the input audio level is within the specified range and add adequate supply decoupling near the VDD pin. Use the standard crystal or external clock as documented in the datasheet to maintain filter accuracy, and adjust the external RC for the desired guard time and minimum tone duration.
This page synthesises distributor pricing, drop-in alternatives, pinout, and practical design notes not found in the manufacturer datasheet alone, helping engineers select the right DTMF receiver for new and legacy designs.
Drop-in alternatives for MT88L70AS1 — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
MT88L70ASR1
✅ Drop-In📋 Reference alternative (not in catalog)
MT8870DSR1
✅ Drop-In📋 Reference alternative (not in catalog)
MT88L70AS1G
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
MT88L70AS1 Specifications
| Product Type | DTMF Receiver (Tone Decoder) |
| Supply Voltage (VDD) | 3 V |
| Input Amplifier | Internal gain-setting amplifier |
| Filter Type | Switched-capacitor bandsplit filter (high/low group) |
| Decoder Type | Digital counting decoder, 4-bit code |
| Tone Pairs Supported | All 16 DTMF pairs |
| Guard Time | Adjustable |
| Inhibit Mode | Yes |
| Power-Down Mode | Yes |
| Functional Compatibility | Zarlink MT8870D |
| Package | 18-pin SOIC (SO-18) |
| Mounting Type | Surface Mount |
| RoHS Status | Pb-free (per distributor listing) |
MT88L70AS1 Pin Configuration
| Pin 1 | IN+ — Non-inverting analog input to the gain-setting amplifier |
| Pin 2 | IN- — Inverting analog input to the gain-setting amplifier |
| Pin 3 | GS — Gain-setting pin (external resistor to VSS sets amplifier gain) |
| Pin 4 | VREF — Reference voltage output (decouple to VSS) |
| Pin 5 | VSS — Negative supply (ground) |
| Pin 6 | OSC1 — Clock input / crystal connection 1 |
| Pin 7 | OSC2 — Clock output / crystal connection 2 |
| Pin 8 | VDD — Positive supply (3V nominal) |
| Pin 9 | St/GT — Steering / guard-time input (RC sets guard time) |
| Pin 10 | PD — Power-down input (active high, places device in low-power state) |
| Pin 11 | INH — Inhibit input (active high, masks tone detection) |
| Pin 12 | Q4 — Decoded output bit Q4 |
| Pin 13 | Q3 — Decoded output bit Q3 |
| Pin 14 | Q2 — Decoded output bit Q2 |
| Pin 15 | Q1 — Decoded output bit Q1 (LSB) |
| Pin 16 | Std — Delayed steering output (goes high on a valid DTMF tone) |
| Pin 17 | ESt — Early steering output (goes high when a tone is detected) |
| Pin 18 | TOE — Tri-state output enable for Q1-Q4 (active high) |
Typical Applications
MT88L70AS1 is suitable for 7 applications: Telephone Answering Machine DTMF Decoder, Security Alarm Panel with Phone-Line DTMF, Remote Control Over Audio Channel, Two-Way Radio Signalling Decoder, Industrial Audio-Band Command Decoder, VoIP Adapter with PSTN Fallback Signalling, Home Automation Phone-Line Controller.
Telephone Answering Machine DTMF Decoder
The MT88L70AS1 is well suited to telephone answering machines that decode remote-control DTMF commands (playback, skip, delete) from the audio line. Its 3V single-rail operation avoids the need for an extra LDO on a 3.3V system, while the internal gain-setting amplifier lets it connect directly to the line-coupling transformer with only a few external Rs and Cs. The 4-bit decoded output plus Std late-strobe can be wired directly to a host MCU, simplifying firmware. The part's adjustable guard time and inhibit mode let the designer ignore transient tones and ambient speech, reducing false triggers in real-world line conditions. Central-office-quality DTMF decoding preserves compatibility with automated IVR systems used in voicemail and remote-access applications.
Recommended
Security Alarm Panel with Phone-Line DTMF
Alarm panels that use the PSTN for back-up signalling rely on a robust DTMF receiver to decode commands from the central station. The MT88L70AS1's central-office-quality decoding and adjustable guard time ensure that arm/disarm or test codes are received reliably even on noisy lines. The internal gain-setting amplifier accepts the wide dynamic range of a telephone line directly, reducing the external BOM. Power-down and inhibit modes let the panel keep the receiver in a low-current state between polling intervals, an important consideration for battery-backed panels. The 18-pin SOIC package is easy to hand-prototype and is well supported by the long-life Microchip supply chain.
Recommended
Remote Control Over Audio Channel
Industrial and consumer remote-control links that overlay DTMF tones on an audio carrier (FM, two-way radio, voice-grade audio) need a low-voltage DTMF receiver on the receive side. The MT88L70AS1's 3V operation makes it a natural fit for portable, battery-powered receivers where the audio demodulator and host MCU already run at 3.3V. Its 4-bit parallel output simplifies the host interface compared with serial-only devices, and the Std signal gives a clean interrupt on a valid tone. The switched-capacitor band-split filters tolerate the moderate frequency error of low-cost audio paths, keeping the false-decode rate low in real-world deployments.
Recommended
Two-Way Radio Signalling Decoder
Two-way radio systems that use DTMF for selective calling (Selcall) and status reporting depend on a DTMF receiver that can ignore voice and noise while still decoding legitimate tones. The MT88L70AS1's adjustable guard time lets the radio designer tune the trade-off between false-trigger immunity and minimum-tone duration, and the inhibit pin allows the host to blank the receiver during transmit. The 3V supply aligns with modern handheld-radio power architectures, eliminating the local boost converter that a 5V DTMF receiver would require. The 18-pin SOIC package is convenient for hand-soldered prototyping of radio accessories.
Recommended
Industrial Audio-Band Command Decoder
Industrial command-over-audio systems (page-and-answer, intercom override, audio-CONTROLLED machinery) need a DTMF receiver that can decode 16 tone pairs reliably in the presence of plant noise. The MT88L70AS1's switched-capacitor band-split filters provide steep roll-off between high-group and low-group tones, while the digital-counting decoder rejects single-frequency and out-of-band disturbances. The internal gain-setting amplifier with external resistor sets the input sensitivity without a separate op-amp. The 3V single-rail operation simplifies integration into industrial controllers that already include a 3.3V rail, and the SOIC-18 package is robust for hand-rework on the factory floor.
Recommended
VoIP Adapter with PSTN Fallback Signalling
VoIP adapters and ATAs that retain PSTN fallback or local DTMF signalling need a DTMF receiver that can decode in-band tones with central-office quality. The MT88L70AS1's 4-bit parallel output plus Std late-strobe can be read by a small host MCU that converts the decoded digit into the appropriate VoIP or PSTN signalling event. The 3V supply aligns with the 3.3V rails common in VoIP adapters, and the inhibit mode lets the firmware mask the receiver during signalling playback to avoid self-triggering. The part's long-life status and Microchip's continued manufacture make it a safe choice for long-lifecycle ATA designs.
Recommended
Home Automation Phone-Line Controller
Home-automation controllers that accept DTMF commands over a phone line (for remote arming, lighting, or appliance control) need a low-voltage DTMF receiver that can run from a wall adapter or battery. The MT88L70AS1's 3V single-supply operation, central-office decoding quality, and adjustable guard time provide reliable digit detection on residential lines with varying noise and level. The internal gain-setting amplifier reduces the external BOM, while power-down and inhibit modes reduce standby current when the controller is not actively decoding. The 18-pin SOIC package is easy to hand-prototype, an advantage for low-volume home-automation products.
Recommended
Recommended Products Summary
Engineering reference data for MT88L70AS1 — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | MT88L70ASR1 | MT8870DSR1 | MT88L70AS1G |
|---|---|---|---|---|
| Package | 18-pin SOIC (SO-18) | 18-pin SOIC (SO-18) - same | 18-pin SOIC (SO-18) - same | 18-pin SOIC (SO-18) - same |
| Brand | Microchip Technology | Microchip Technology | Microchip Technology | Microchip Technology |
| Supply Voltage | 3 V | 3 V | 5 V | 3 V |
| Function | DTMF receiver, 4-bit parallel output | DTMF receiver, 4-bit parallel output | DTMF receiver, 4-bit parallel output | DTMF receiver, 4-bit parallel output |
| Power-Down Mode | Yes | Yes | Yes | Yes |
| Inhibit Mode | Yes | Yes | Yes | Yes |
| Tone Pairs Decoded | 16 (full DTMF) | 16 (full DTMF) | 16 (full DTMF) | 16 (full DTMF) |
| Packaging Format | Tube | Tape & Reel | Tape & Reel | Tube |
| Price (qty 1, USD) | 2.85 | 2.85 | 2.95 | 2.85 |
Key Differentiators
- 3V single-rail operation (vs MT8870D)
- Same-die SOIC-18 tape-and-reel option (vs MT88L70ASR1)
- Internal gain-setting amplifier with adjustable guard time (vs Generic 3V DTMF receiver)
Design Notes
Estimated: the MT88L70AS1 is specified for a nominal 3V supply; place a 0.1 uF ceramic decoupling capacitor as close as possible to the VDD pin (pin 8) with a short ground return to VSS (pin 5), and add a 10 uF bulk cap on the same rail if the supply is shared with noisy digital logic. Use the PD pin to gate the receiver in standby and pull INH high during transmit or signalling playback to prevent self-trigger.
Keep the analog input traces (IN+, IN-, GS, VREF) short and away from the digital outputs (Q1-Q4, Std, ESt, TOE) and from any switching regulator or clock trace. The crystal or external clock on OSC1/OSC2 should be placed adjacent to the IC with the load capacitors tied to a clean analog ground. If the input amplifier is configured for high gain, route IN+ and IN- as a differential pair to reject common-mode noise from the telephone line.
A common mistake is to drop in the 3V MT88L70AS1 onto a 5V board designed for the MT8870D - the VDD rating is 3V, so the 5V supply will exceed the absolute maximum. Also note that the 20-pin SSOP variant MT88L70AN1 is functionally similar but is NOT pin-compatible with this SOIC-18 part; do not substitute the two without PCB rework. Finally, if the application requires more than the MT88L70AS1's 4-bit parallel output, add a small MCU to convert it to UART, SPI, or USB.
Estimated: the switched-capacitor band-split filters require a stable clock on OSC1/OSC2; an external 3.579545 MHz crystal is typical. If the clock is noisy or off-frequency, the high-group and low-group filter centres shift and the decoder will reject valid DTMF tones or accept false ones. Verify the oscillator start-up margin under all temperature and supply conditions and avoid injecting digital noise into the OSC1/OSC2 traces.
Compliance Information
Pb-free per distributor listing (DigiKey part description: 'Pb Free'); REACH, halogen-free, and conflict-minerals status not in the verified web data - listed as 'unknown'.
