Electronics > Metrology

AD7172-2 Noise tests.

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MasterT:
 Subject of my research is oulined here:
https://www.analog.com/en/analog-dialogue/articles/why-does-voltage-reference-noise-matter.html

 Short summary:
1. Noise measurements of the ADC with input pins shorted - absurdity.
2. Simple LPF using ultra low leak polarized electrolytic capacitors is very efficient way to filter internal /external V-ref.

Tests results.
   Input always 2V, filtered low noise source obtained with help 10k & 1.500 uF RC network. Internal adc buffers active for both,
   IN & Ref-IN.
1. V-ref selected internal - 2.5V, using adc register 0x20 setiings. RMS value: 10-11 (LSB)
2. V-ref selected external - 2.5V, switch on, low time constant. RMS value: 5-6 (LSB)
3. V-ref selected external - 2.5V, switch off, big time constant. RMS value: 1-2 (LSB)

For reference, noise with input pins shorted ~1 LSB.

Kleinstein:
The noise with a shorted input still makes sense, as one point for the noise testing. This however only applies with a short before the input buffers, not the ADC in unbuffered mode and than an short.
Reference side noise may well be more important with a voltage near the full scale. Still quite some measurements would be done with way less than full scale.

The filters used for the reference and input look like they could effect the accuracy. One point here is dirft in the input current and the other point is input current to the filter capacitors. The capacitors react to temperature and mechanical stress and because of the DA settling can take a long time (like a few hours).  The filter helps with the nois, but will reduce the accuracy.

MasterT:
Ref. Input curent with buffers on = 100 nA. Since there is constant voltage & sampling clock frequency - no issue is expected with linearity.
Voltage drop accross 10k resistor = 1 mV
Initial accuracy of the internal voltage reference +-0.12% or +-3mV

As you can see accuracy is not sucrificied.
Capacitor leakage current is ~10 nA or 1/10 of the ref. inputs, the only concern its may rise with temp., than active LPF would be necessary.
Caps tested so far:
UKL0J102MPD
RNL1C152MDS1

Kleinstein:
I see 3 problematic points with the 10 K resistors:
The input current of the ref. buffers has quite some drift:  +-0.25 nA/K typical from the datasheet. With 10 K and a 2,5 V ref. this tanslates to 1 ppm/K of drift. If the buffer is BJT based, there will also be 1/f current noise that can become a problem. The linked article on reference noise showed an low pass filter using the AD797. It works good for the higher frequencies, but at the lower frequencies it gets worse than it was before.

The temperature drift of the capacitor leakage current. 10 nA is quite low, but chances are it is also quite temperature dependent. If it is similar to semiconductor leakace the drift may reach 1 nA/K.

The final problem is the DA if the capacitor. With electrolytic capacitors this can real a few percent and is thus needs quite along time to settle after turn on. To get settling to the ppm level it would need some 1000-10000 RC time constants and with the low cross over frequency this gets really long, possibly more than 1 day. Here it is not so much the resistor itself that is the problem, the point of having a long time constant with an electrolytic capacitor.
It is a bit hard to tell appart the settling of the capacitors leakage current and DA. For the capacitor leakage (maybe DA) it is no do uncommon to take days for settling.

Filtering the low frequency reference noise is a tricky thing. Even film capacitors reactor to thermal disturband and stress / seismic events. A some point the effort for filtering gets larger than simply using a lower noise or 2nd reference chip.

This is anyway a topic seprate from the ADC itself, but more thing for the reference. For testing the ADC it should be good einough to have the same reference for the ADC and test signal. In this case there is not need the low frequency filtering - just the higher frequency filtering can be relevent.

MasterT:

--- Quote from: Kleinstein on May 31, 2023, 04:56:06 pm ---This is anyway a topic seprate from the ADC itself, but more thing for the reference. For testing the ADC it should be good einough to have the same reference for the ADC and test signal. In this case there is not need the low frequency filtering - just the higher frequency filtering can be relevent.

--- End quote ---

 It's all about ADC noise tests with respect DC input voltage measurements, when in general case input signal is not correlated to reference voltage in any way. So situation is just opposite, low frequency noise is matter, high frequency has no meaning for SD ADC at all.


Indeed settling time of the filter is quite long, so I came up with simple circuits to disconnect capacitor during power outage..
Recorded voltage lost is 60 mV when circuits was w/o power for ~10 hours.

 

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