Hello
I'm getting an offset error when I sample a signal 3 times in a row. The ADC I'm using is a ADS8341. All three samples get different offsets.
The input to the ADC is driven by an OP.
My reference voltage is 3.000 V and the offset difference is 0.5mV.
What can I do to get rid of this offset error?
Are the offsets random? If so, that's "noise", not offset.
If the three samples always progress in the same direction, you may be injecting charge into the opamp and piling up transient offset errors. If that's the case, an R-C lowpass between the amp and the adc may help.
1 part in 6000 is pretty demanding; likely noise in the signal, in the reference, in the amp, or in the ADC.If it's noise, nothing, really. You can take a lot of samples and average or lowpass filter, trading bandwidth against noise.
Got more details?
John
Or a good fast op-amp in a circuit that gives you a fast response and a low-impedance output _at the frequency of interest_ (most amplifiers built around an op-amp have disappointing output impedances at high frequencies because of gain rolloff -- you have to compensate for this).
Do you mean op-amp?
Sometimes it's best to sample at the very fastest that the ADC will do, to get you more samples to average over -- ADC front-ends have very wide bandwidth, and don't seem to be optimized for noise, so it is often ghastly.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
The problem is solved.
I measured input voltage to channel 0 (Vch0). The Vch0 showed a transient at the sampling point. I put an RC filter between the OP-amp output and the input channel 0. Then the transient was gone. But I still had the offset problem between the 3 samples.
Then I measured the Vref to the ADC. The Vref showed some fluctuations between the 3 samples. So I put an extra capacitor across it and a series resistance (an RC filter). And now the offset diffenrence is almost gone.
So I need to make the Vref more stable to get it more perfect.
Tim Wescott skrev:
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