AD73360L - Offset

Hi Quintin,

     I'll move this thread to the Energy monitoring and metering community.  Someone here should be able to respond to your inquiry.

Regards,

Jonathan

Hi Quintin,

Sorry about the delay, since this was originally posted in a different section we weren't directly notified. I will help you out with this problem.

First off, is this still an issue?

If it is, then my next question is, are you using CT's as the current sensors in the application?

From the description of the issue, it sounds like it could be a capacitor that is the problem. In particular C38, 39, 42, 43, 46, 47, 50, 51, 54, 55, 58, and 59, all the capacitors in series with the input signal path. One good test for that would be to short VINPx and VINNx, to see if the offset still exists. Or by shorting out the caps listed, this should be ok if Current Transformers are used as the current sensor.

Regards,

David

Hi dlath,

Thanks for the reply, sorry for only replying now. I didn't set the discussion up to notify me.

We are using CTs but we have taking everything out of the equation by shorting the  VINPx and VINNx like you mentioned too and the offsets still existed.

We have found anomalies in the BF51xF's datasheet to do with the SPORT port but we are not entirely sure if they are what is causing the offset. We have also done a new PCB layout where we moved the ADC much closer to the SPORT on the DSP and also improved the layout a lot. We are waiting for our PCB's to be assembled and I will then get back to you on how it performs.

We did pick up that touching the SPORT lines did induce noise on the data. So basically we noticed that the SPORT had an effect on the data. Digital noise being introduced.

Kind regards,

Quintin.

Hi dlath

The new PCB's with the ADC closer is a bit better but still has an offset when the temperature changes. I think we need to engage on a different level. Can I maybe send you a copy of the PCB layout and schematic? It feels like we are missing something here and we do need help from Analog.

Kind regards,

Quintin.

Hi Quintin

I can fill-in for dlath and help you further.

I looked at the above posts and I have a few questions for you:

1. When you mention 'offset', I am assuming you mean DC offset (or do you mean inaccurate results)? The DC offset is specified to be within +30mV over the entire temperature range from -40C to 85C, at PGA=0dB mode. Are you saying that the DC offset for certain ADCs after power-up is more than this limit?

2. I can see that the DC offset varies over time in your case, which is definitely not expected. How much is this variation? How much variation over temperature do you see? Can you provide some numbers here, like average ADC code in each case?

Can you attach the PCB layout and schematic here? I will review it and get back to you.

Regards

hmani

Hi ,

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?

Kind regards,

Quintin.

Hi Quintin

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.

Regards

hmani