Using a chopper to measure microvolt-level (tiny) DC signal with PCM1808 - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: April 15, 2025, 07:30:41 pm
content: I'm interested in using PCM1808 (24 bit stereo ADC with I2S interface used in audio equipment) to measure DC signal that is potentially very small (coming from a diode detector fed from a small RF signal.)j PCM1808 because it's very inexpensive for a 24-bit ADC.The PCM1808 isn't intended to measure DC levels, and output of the detector is a (potentially very small - microvolts) DC signal.My thought is to process the DC signal through a chopper (I have some J111 FETs on the way that were marketed for chopper applications.)I've never designed/built a chopper before, but the general concept seems simple enough, and the PCM1808 would end up being fed by an AC signal (square wave).Looking for pointers to example designs or discussions of chopper circuits used for this sort of application.Thanks!

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 16, 2025, 03:02:57 am
content: This is not a good idea.The PCM1808 is not an accurate ADC. It's designed for audio, and audio applications do not care whether the full scale voltage is 1V or 1.01V p-p. The only goals are low distortion and low noise. (Basically, the digitized signal can be larger or smaller than actual). The reason a huge number of bits are needed is to lower the quantization noise. If you feed in a signal with a 1 kHz and 2 kHz sine wave, the ADC can tell you with high accuracy what the relative strength of the two signals are, but it can't tell you accurately what the actual power is for the frequencies. The noise floor will also be very low (because there is almost no quantization noise with 24 bits of resolution). Notice how the datasheet mentions nothing about offset, voltage reference stability, temperature stability, etc.(This is also a very common trap for DACs too which I have fell for in the past. A 16-bit audio DAC costs pennies, but writing all zeros to the device leads to an output of about 0.7V and writing all ones to the devices leads to an output of 2.4V for a 3.3V device. The DC levels are not accurate, and the output swing of the DAC is also not accurate. If you fed in a sine wave digitally, then your analog output would be a low distortion sine wave with an inaccurate amplitude. Maybe the sine wave could be 1V or 1.03V p-p, but the sine wave will have low distortion and that's all that matters for audio purposes. (The user can just twist the volume potentiometer to compensate).If you want a step in the right direction go look at how the HP 8481 power meters are implemented. The HP 8481 measures a nanovolt difference generated by a thermocouple caused by the RF power entering the sensor (which can be -30dBm). You can also just buy a chopper IC like the MCP6V51 (see this:https://github.com/jaromir-sukuba/nvm)

author: wb0gaz
author_rank: Frequent Contributor
timestamp: April 17, 2025, 07:56:24 pm
content: Thanks - that's helpful to know.Are these types of devices (audio ADC) repeatable? That is, if quiet yields 0.7V measured and loud yields 2.5V measured (by looking at the relative amplitude of the digitized signal), is that range repeatable (that is, if I do this 100 times, will I get the same quiet-to-loud difference), or are there drift concerns as well?My concern wasn't to get absolute amplitude accuracy, but to get resolution and repeatability, so I could begin by calibrating the detector/ADC measured amplitude (with a known RF source level) then be able go back in the future and compare an incoming signal against that "calibration" point.I have studied the chopper design in the HP sensors; that is what gave me this idea in the first place.The sensor (in this case) would be a Wiltron 560-series scalar network analyzer sensor (a diode detector with an embedded thermistor to report temperature).

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2025, 08:12:15 pm
content: The audio ADCs tend to have quite some gain drift. So the gain changes with temperature. Another issue is the I2S interface that can be a bit picky with the clock and limited with the sampling rates.There are alternative ADCs made for power metering and similar jobs, that are more stable and still affordable. The MCP3561 is such an example.For a µV signal the ADC noise is anyway not that relevant. Usually the amplifier part and maybe thermal EMF and EMI will dominate.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: April 17, 2025, 08:19:59 pm
content: OK thanks for the helpful information!Looking now at MCP3561 datasheetDave