Back to Basics: ADC and Impedance

Posted under Back to Basics, Teardown, Tech Explained on August 24th, 2025 by

Recently, in a discussion with a few folks, I found that ADC concepts were not that clear. Esp. with regard to the source impedance part. Some were under the impression that you connect a voltage within ADC range and measure, thats it. This might be a very basic thing, but I thought I should explain.

ADC Impedance

Source impedance is simply the open-circuit resistance seen looking back into whatever is driving the ADC input. An ideal source is zero ohms. A real sensor, voltage divider or source output has some finite output source resistance. That resistance, the ADC’s input resistance and capacitance form an RC low-pass filter. Check images for the equivalent circuit.

ADCs have a sample‐and‐hold capacitance (Approx tens of pF). When a voltage source is connected to ADC, this capacitor charges through the source resistance and an internal switching resistance (approx 1-2kΩ). This RC circuit needs a finite time to rise up. If you have a 2bit ADC for the last bit to settle, it would take approx 10x the RC time constant. To put it approx numbers, let’s say Rsource=1kΩ, Ron = 1kΩ, C= 10pF, RC time constant is 20ns, 10x that would be 0.20us. So you need to sample and take values only after this settling time for the LSB, else it will give you are wrong reading. This becomes a major factor if you are taking only 1 ADC reading of the sample and then going away. If that value is not settled, you get a wrong reading. Especially applicable when sampling fast changing AC signals. Now if Rsource is larger, say from a large MΩ voltage divider, your sampling time is going to 1000x this number. So always aim to keep source impedance less than 1k or so if you can afford the current.

What to do if you can’t? Then use a buffer so that your flow will be,
Sensor ->(High-Impedance) -> Op-amp (Unity gain buffer) -> (Low-Impedance) -> ADC input. 
This guarantees the ADC always see a low-impedance source because of the op-amp and your input is effectively shielded.

Please do take care of the source impedance case. There is lots to discuss regarding ADCs. If there is genuine interest, I can spend some time creating a series in the future. Do let me know.

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Back to Basics: Piezos

Posted under Back to Basics, Tech Explained, Technology on August 10th, 2025 by

Earlier this week, I was in a discussion with a client on piezo electric tech, especially about 2 pin and 3 pin types. I thought it’s worth a quick post to clear the most common confusions and when I would pick each one.

Piezos

Simply put, a piezo unit is a thin ceramic disc with electrodes on both ends stuck on a metal plate. If you apply a DC, it deforms in one direction and if you reverse it deforms the other way. Now, if you apply AC, and it flexes fast, pushes air, and makes sound. Electrically, it’s just a small capacitor with a resonance in the few kHz range.

Piezos

What I explained above is a passive 2-pin piezo. It’s a transducer disc with two electrodes. You must drive it with AC or PWM near resonance frequency of the disc. There is another type called the 2-pin active piezo. It has a built-in oscillator. Give it DC and it beeps at a fixed tone. Putting DC into a passive disc won’t make a sound.

3-pin piezos are in two options. One is a three-electrode (bimorph) disc. It has an extra terminal called a feedback electrode. You can build a simple loop (check images) that senses the disc’s motion and feeds it back to sustain oscillation. The loop naturally locks to the true resonance of the disc and produces the loudest sound (So it’s used in alarms). There is no firmware or MCU involved so it is cheap. Since it’s “auto-tuned”, even changing enclosure will affect its sound output slightly.

The other is a 3-pin module. Pins are VCC, GND, and SIG. The element inside is still two-pin; a small PCB adds a transistor so an MCU pin can switch it to produce sound.

Anyways, everything above was about using piezos to create sound by pumping energy in and flexing the disc. The reverse is sensing. The same ceramic generates a voltage when you press, bend, or tap it. That means you can detect knocks, vibration, even airflow, and in some cases harvest energy from motion. I’ll keep this post focused on buzzers for now, but we can dive into piezo sensor front-ends, buffering that high impedance, filtering etc in the future.

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