08-03-2010 05:54 AM
I have a PCI-6030E DAQ board where the AI signals are connected to the output of 16 individual instrumentation amplifiers (AD620). The incoming signal to the amp is always -6.87 volts. The output when the 6030 is not acquiring data is -6.87 volts. When the 6030E is acquiring data across all channels (16) at 2,000 hz the measured voltage is -6.87 volts. When the sampling rate is increased to 5,000 hz across all channels, the measured voltage is -6.26 volts. The 6030E is configured in RSE, with the input range set to -10 volts minimum, and 10 volts maximum. The AI ground signal is connected to the amplifier output reference pin, so essentially I have a differential input to single ended output converter if everything is working properly... Attached is a schematic of the interface board. Question is, why would the measured voltage change at higher sampling frequencies? Note: at 5,000 hz, the AI clock rate is at 80,000 hz, this is below the maximum AI clock rate of 100,000 hz...
08-04-2010 10:59 AM
I wanted to clarifiy a couple things with your post.
1. Are you connecting only one signal at a time while performing your measurment?
2. Do you exhibit the same behavior if you input -6.87 V to each instrumentation amp?
3. Do other channels exhibit unexpected behavior if you are providing input to just one instrumentation amp at a time?
08-04-2010 06:07 PM
Thanks for the reply.
At the moment I have 5 signals connected. These 5 channels also exhibit some amount of change at high ACQ rates, but significantly less than the channel connected to the -6.87 V signal (probably because the signals are less than ±1 volt to begin with). Although, the other channels have some amount of change, I haven't had the chance to quantify the amount yet. My big concern has been the -6.87 V channel, since I am implementing a control system about the -6.87 V transducer signal.
It is possible that the inputs for the unconnected channels are saturated. I will try putting a 10k resistor across the input to the ad620 op-amps that are not being used. Do you think that a saturated channel on the input to the 6030 board could be causing the odd behavior at the higher sampling rates?
08-05-2010 03:45 PM - edited 08-05-2010 03:47 PM
I am slightly concerned you may be suffering from some sort of ghosting effect. If that is the case you can try a few things:
I know you may have done some of these things already I just wanted to add them for the sake of being complete. Let me know the results!