Chuting

Hi, I am using a Nexys video with another adapter card, which requires 2.5V from FMC connector. Currently I follow what the reference manual(page 6) says and set the control signals one by one in the stated order to set the FMC bank voltage to 2.5V: VADJ_EN(low)->SET_VADJ(2'b10)->VADJ_EN(high) giving ~200ms between each step using a counter. I put this VADJ setting part before my main module (reset is deasserted when VADJ setting is done) and the whole thing works fine. But I am wondering if this VADJ setting sequence is really necessary and if I can just assign SET_VADJ=2'b10 in the beginning while VADJ_EN is high. My guess as a beginner is that feedback voltage of the DC-DC regulator (ADP2384) becomes higher/lower than what it should be when SET_VADJ is changed while regulator is ON. It enters protection mode (output off) then the recovery time is hard to tell and it is not clear when to start the main module. Is this one of the reasons why VADJ needs to be set in this particular way? or are there other reasons? Thanks!

Hi John, We tried measuring 4.8V from 3 batteries in series with MCC128 (single ended mode, +/- 5-volt range, after 30mins warm-up) and the result was great. As shown in attached figure, the standard deviation was ~0.095mV. The accuracy was also good. For accuracy comparison between MCC128 and calibrated DMM, maximum difference was <1.0mV within 20000 samples each (no averaging). So it turned out MCC128 is working perfectly fine. Its precision and accuracy are well within the stated specs. Instead, the problem came from the source we measured previously. The other data logger used to verify the precision probably has some built-in noise filters, which users can’t turn off. So at first, we interpreted the data fluctuations as MCC128’s fault. We will consider adding a low-pass filter before inputs. It was nice learning batteries can be used as stable testing sources. Thank you so much for your help!

Hello John, Thank you for your suggestions. We tried those out but unfortunately they didn't make much difference. We measured the 3V signal with a calibrated multimeter and confirmed the standard deviation is ~0.2mV (attached "3V_source_multimeter.jpg"). Measurements with MCC 128 and taller headers(board-to-board distance 25mm) showed data fluctuations similar to those with original headers, standard deviation ~7.2mV (attached "MCC128_25mm_distance.JPG"). It seems that the data (20000 samples, 1kHz sampling rate) does not have much 60Hz component(attached "FFT_data"). Sampling @ 60Hz didn't improve the noise. The serial number is DB5E8B7 as shown in the attached image. Thank you for your help, Chuting

Hello, We hope to measure voltage of a laser sensor with MCC 128+Raspberry Pi 4 but found out the measurement precision is way worse than what we want to achieve. To test the precision, we measured 3V DC voltage source with claimed ripple noise Vrms ~0.5mV and acquired 20000 samples with 1kHz sampling rate. The data shows standard deviation of ~7.7mV. The same source was measured with a different data logger (another maker, without filters) and standard deviation was ~0.19mV so the noise shouldn't come from the source. Attached are the python code for this test and measured data (ch1: 3V DC measurement, ch2: AGND measurement, ch3: open). Are there any ways to improve the precision? Or is this what we should expect from MCC 128 boards? We are new to analog to digital converters and any ideas would be appreciated. 3channelADC_trigger.py 1kHz_20000S_test_data.xlsx