Electret Microphone to TI ADC Interface Circuit. - Page 1
Forum section: electronics / projects

author: bw459205
author_rank: Newbie
timestamp: May 05, 2025, 10:49:04 pm
content: Dear friends,I would like to interface four electret microphones to a Texas Instruments TLV320ADC6140 ADC chip. My question is, what is the best way to go about this for optimal performance?Should I use a pre-amp circuit, or can I bias the MIC and connect directly to the ADC via AC coupling capacitors?Additional context: All four microphones will be mounted to the same PCB as the ADC low noise / high SNR is a key factor... Achieving low phase distortion is even more important. I would also like to minimise size and cost, so component count is a consideration Required bandwidth - 50 Hz to circa. 16 kHz. I don't want to use an OpAmp because of point  I don't want to use MEMs as the SNR is inferior to electret currently.In summary, should I use a transistor preamp circuit, as shown in the attachments, or can I connect directly to the ADC?Note that the ADC datasheet example shows MEMs microphones connected without preamps. These are presumably low output impedance.What do you reckon experts?Thx!

author: moffy
author_rank: Super Contributor
timestamp: May 05, 2025, 11:37:50 pm
content: You need to specify what your maximum gain is and whether it needs to be variable. Another point is distortion levels since you seem to be comparing chanels, opamps will generally give lower distortion, and a quad opamp will tend to have somewhat matched characteristics between them since they are on a single die.

author: bw459205
author_rank: Newbie
timestamp: May 06, 2025, 10:25:41 am
content: Thanks.It can be unity gain in the preamp because the ADC chip has a programable gain block.

author: moffy
author_rank: Super Contributor
timestamp: May 06, 2025, 12:22:08 pm
content: You don't really need a buffer or amplifier, as long as you are happy with the signal level, and the gain the PGA can provide, page 28/figure 37 of the datasheet:https://www.ti.com/lit/ds/symlink/tlv320adc6140.pdf?ts=1746533600225&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTLV320ADC6140Shows how to connect a single ended source such as your microphone to the ADC via capacitors, one to GND and the other to the output of the MIC.

author: bw459205
author_rank: Newbie
timestamp: May 06, 2025, 12:58:15 pm
content: Thanks moffy.The input impedance of the ADC is relatively low (2.5 kOhm by default), and the output impedance of the microphone will be a few kOhm.Presumably if I connect them directly without a buffer I will lose some of the signal amplitude? Which could be then recovered by setting a higher gain in the PGA block inside the ADC.This approach is OK for me as long as I can achieve minimal noise and distortion.

author: moffy
author_rank: Super Contributor
timestamp: May 06, 2025, 11:27:37 pm
content: Quote from: bw459205 on May 06, 2025, 12:58:15 pmYou can select an input impedance of 2.5k, 10k or 20k, which affects the input noise slightly, but since you have an AC coupled roughly 2.2k driver, the input inpedance will be that in parallel with the ADC impedance, and the noise will be proportional to that lower impedance. So choose the higher impedance 20k and you will get low noise and minimal attenuation.

author: TimFox
author_rank: Super Contributor
timestamp: May 06, 2025, 11:35:20 pm
content: The output impedance of the actual electret microphone is capacitive, and your roughly 2k impedance is that of a built-in amplifier.(Otherwise, the cable capacitance would give very bad loading of the microphone at high frequency.)The noise of the system is probably dominated by that built-in amplifier (typically, a simple JFET stage), and it may well contribute substantial distortion at very high sound levels.What's wrong with op amps? The best ones have decent noise from these impedances, and it is hard to get better distortion results than a quality IC.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2025, 01:09:54 am
content: Quote from: TimFox on May 06, 2025, 11:35:20 pmA post JFET amplifier/buffer won't remove the noise or distortion of the JFET buffer, unless you have something else in mind? The JFET buffer adds a small amount of distortion because the signal level is so low.

author: TimFox
author_rank: Super Contributor
timestamp: May 07, 2025, 01:16:21 am
content: Quote from: moffy on May 07, 2025, 01:09:54 amYes, the post-amplifiers will not remove the noise and distortion of the first amplifier.However, a properly designed op-amp stage should not add substantial noise or distortion for this purpose, and is easier to build than a discrete-device design.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2025, 01:28:51 am
content: Quote from: TimFox on May 07, 2025, 01:16:21 amThat is certainly true, that's why the suggestion is to AC couple the output of the electret directly to the ADC as per Figure 37 of page 28 of the datasheet. It seems the OP is happy that the internal PGA has enough gain for the signal.

author: TimFox
author_rank: Super Contributor
timestamp: May 07, 2025, 02:16:31 am
content: I was reacting to point  in the first post.

author: bw459205
author_rank: Newbie
timestamp: May 09, 2025, 10:39:21 pm
content: Thank you guys for your responses.I have raised a ticket with TI on this (who make the ADC).I agree it is certainly possible to make either discrete or OpAmp buffer stages for this application.My confusion comes from the datasheet in that the only example showing a direct connection is a MEMs microphone which have internal buffer stages, and are hence low output impedance.I want to know if it is inadvisable to do the same thing with a biased ECM directly? I am building acoustic phased arrays. The inter-channel phase errors matter, so I want to give it the best chance of success.Lets see what TI say on this.Cheers.

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2025, 10:44:38 pm
content: Since the electret microphone element is a variable capacitance, the first amplifier stage must be inside the package that includes the element. That stage should have a reasonably low output impedance.The original voltage-biased condensor microphones used a vacuum tube to load the element with a small constant capacitance and a very high resistance: FETs are more common now.Otherwise, the high-frequency output would be a strong function of the cable capacitance, and the low-frequency range would suffer from a serious high-pass filter response unless the next input resistance is huge.

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2025, 12:11:36 am
content: Quote from: TimFox on May 07, 2025, 01:16:21 amThe "buffer" inside the microphone is a common source JFET with the output connected to the drain, making it a transconductance amplifier. The variation in voltage across the electret microphone element is directly impressed across the gate and source to produce a variation in current at the drain. Since the drain impedance is high, the load resistance sets the output impedance, minus capacitive effects at high frequencies.Voltage noise at the gate produces current noise at the drain. Voltage gain, set by the load resistance, amplifies both the signal and the noise, so signal-to-noise is roughly fixed as the load resistance varies.There might be some magic value of load resistance where distortion from drain voltage variation cancels out, but otherwise I would assume distortion is better with lower load resistances. This suggests lowering the load resistance and making up for the lack of gain with a low distortion feedback amplifier. Maybe measure the output noise at 1kHz to get an idea of what kind of preamplifier input noise level is required.Modern operational amplifiers are plenty quiet. In the past I suspect parts like the ubiquitous but noisy TL071 used in audio applications were considered too noisy, leading to discrete input microphone amplifiers. Parts like the NE5532 could have been used but were much more expensive.