# Source Impedance Consideration for ADC

I recently used accelerometer ADXL335 in my academical project, this sensor gives analog output. I interfaced this sensor with microcontroller PIC18F458 using ADC module. It was successfully interfaced and shown correct readings. But later I noticed in datasheet of controller that "The maximum recommended impedance for analog sources is 2.5 kΩ" and datasheet of ADXL335 shows that its output impedance is 32kΩ. I have not used any signal conditioning circuit between sensor and controller. I am wondering how I am getting correct output even if source impedance requirement for ADC module of controller is not satisfied.

• How often do you take a measurement, or, in other words, how much time are you giving the ADC for each measurement? Oct 12 '20 at 10:50
• Data sheet links and schematics please. Oct 12 '20 at 10:56
• If you have the datasheet-suggested 100nF capacitors on the accelerometer outputs, then the impedance is not 32kohms but much lower. Oct 12 '20 at 11:11

## 2 Answers

The source's output impedance (and capacitance) will affect the settling time in most ADCs.

An ADC can (depending on the type) do a good measurement of higher-impedance sources if you give it more time (to charge the sampling capacitor); the more time you give it for a measurement, the higher the source's output impedance can be. Adding a small capacitor will also let the ADC work properly with higher impedances.

The recommendation is as it may be, but doesn't mean measurements will fail if done properly, taking the source's impedance into account.

So, my answer would be that it is working properly because you give the ADC plenty of time for a measurement, and/or you have added a capacitor on its input.

• I haven't added external capacitor but yes there was few seconds time between two readings. Oct 12 '20 at 12:19
• It's about how much time (how many clock cycles per sample) you allow the ADC before getting the measurement, not so much about the time between measurements. I don't know how this is set for your specific ADC module, or if it can be set at all, though. Oct 15 '20 at 11:24

Here's probably what made it work fine: -

The text says that capacitors must be added and presuming that you have, then the source impedance required by the ADC will become more relaxed. Pretty much most ADCs of this type can tolerate several tens of kohm source impedance if the capacitors fitted are circa 100 nF. There should be details in the MCU data sheet that explain this.

If you haven't used capacitors then you should.

Regards the input leakage current of the MCU, the maximum value is 500 nA and this will create an offset error (due to the 32 kohm source resistance) of 16 mV. If you can live with this (it affects all readings a constant amount) then that's fine. If you find that this DC error is problematic then you might need to buffer the signal with an op-amp. At normal operating temperatures the leakage will be about one-tenth of 500 nA so, is a 1.6 mV DC offset a big deal to you?

• I haven't used capacitors also still getting output (I agree it is not 100% correct) and that's why I am surprised Oct 12 '20 at 12:16
• @AKumar Well, if you are not sampling at a high rate you might get away with it but, it's likely that there are signal amplitude issues that you are just not aware of but, your "application" is OK in what it does. Oct 12 '20 at 12:18
• i am using it for tilt angle measurement Oct 12 '20 at 12:22
• Do you pre-calibrate your circuit at various angles before launching it into doing a real job? Oct 12 '20 at 12:30
• yes, i checked output with reference values Oct 12 '20 at 12:34