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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Advanced Tech: PicoLeaf

Posted under Educational, Tech Explained, Technology on August 17th, 2025 by

I only recently learned about the PicoLeaf tech from Murata. It’s a cool bit of tech worth discussing. Last time we discussed piezos and PicoLeaf takes this piezo tech to the next level.

PicoLeaf is a flexible piezoelectric film sensor that can detect bend, twist, press and vibration. It’s thin(0.2mm) and tiny, so it wraps around curved parts. It’s even sensitive enough for µm-level motion, which is great for HMI and biosignal acquisition use without large parts. The film uses polylactic acid. Yes, PLA, the same stuff you use for FDM 3D printing. Murata orients the PLA to make it piezoelectric. It can be mounted on a device with a double-sided adhesives.

Pico Leaf

Like any piezo, it produces charge in proportion to strain rate. We can read it with a charge (I/V) amplifier (with a mid-supply bias) and then add gain and filtering to pass it to an ADC. All the taps/presses get captured nicely via the microcontrollers ADC at reasonable sampling rates. It can detect displacement direction(press/release) and displacement velocity based on signal amplitude. Its non-pyroelectric also, meaning no value drift because of heat. Power consumption is zero as its passive, so only power needed is for the driver portion in µA range.

Pico Leaf

The thin form factor opens up many applications. Since PLA transmits light, it can even sit under clear panels, making it useful for industrial touch displays that require a firm push. Sensitivity may be lower than capacitive touch, since the datasheet does not specify minimum activation force. It is well suited for flexible gloves to detect finger bends or sign gestures. If the claims are correct, it could be a breakthrough in robotic hands for grip sensing. With all the humanoids that planned in the next few years, PicoLeaf would fit right in. Murata’s tests shows it survives 500k bend cycles, which is great.

What I would love to see is whether it can also operate in reverse like a conventional piezo, producing mechanical motion when voltage is applied. With this form factor, that would be a killer feature.

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