We are using the AD73360L as AFE for a industrial product. We have found that if the unit has been left unpowered for +10 minutes and powered up again that there is a offset on one or two of the channels. We investigated this more by putting the unit in a oven to heat it up, where we found that the offset stayed the same. But when cooling the unit and then powering it up, there is a offset on our values. This offset does disappear after the unit is left running for 30-40minutes.
The unit was calibrated both offset and amplitude. We calibrate the offsets by letting the unit run for a minute or two to get the temperature to settle. Then without injecting any current we calibrate the each channels offset by taking the sum of 640 samples and dividing it by 640 to get a average offset value. The ARM doing the calibration also takes a 1000 samples to calculate the average offset value and they correspond.
It is just very interesting to us that the offset is a bit bigger when the unit is left switched off and then switched on (especially only on one or two channels). Then after being switched on for a few minutes the offset drifts back to zero.
We are using the AD73360 in full differential mode and sampling at 32 kHz. We did our layout as close as possible to examples and reference designs we could find on the AD76330. Is there anything we are missing? We first noticed this without cooling the unit in a cooler. But to make sure it is temperature effected we took it to extremes but never more than was specified in the datasheet.
We have changed the anti-aliasing filter and even took it off but got the same offset drift. Once it settles (after 30-40minutes) it stays stable. But then we switch the unit off and leave it off for +10 minutes and switch it back on and the offset is there again.
Any help or ideas would be appreciated.
Quintin van Wyngaardt
Can I send the gerbers and schematic to you directly?
1. Definitely a DC offset. We do current measurements but we pick this DC offset up more easily when injecting nothing and just looking at the ADC values. We are using the PGA = 18dB when doing measurements from 10mA to 1A and PGA = 6dB when doing measurements from 50mA to 5A. The CT's burdens are configure not to exceed the ADC input voltages as you will see on the schematics that I will send to you on a private message.
2. The amount of DC offset is not what is really a problem, it is more the variation as temperature varies that is a problem. We only do a once off DC calibration and that has to last. We cant calibrate the DC offset on the fly although we are now looking into methods of doing that on the DSP. We don't know what to do on the hardware anymore.
3. Do you agree that the DC offset must be stable over the specified temperature range? Thus, once the DC offset is determined for a certain channel, that DC offset will stay the same over the specified temperature and should be constant throughout and can be deducted from the incoming signal on the DSP?
Please let me know how I can send you the schematics and the gerber files on a private level?
I would expect the DC offset to vary over temperature but still remain within the min/max limits. I do not have any temperature drift data to share with you. How much variation do you see? Is it the same level of variation for all ADCs?
You can start a private discussion with me by going to Create-> Discussion on this community's homepage and select Private from the option below the message window. Include my user name there to initiate a discussion with me privately. Let me know if you have any trouble doing this.