Question
I need to measure the DC offset of an audio signal at various points in a
circuit. The signal amplitude can vary between +-12V. Ideally, I would like to
be able to measure offsets with a precision of a few micro volts. I was
thinking of using an ADC like AD7794 to get the resolution and to to have a low
inherent offset, driven by an ADA4528-2 to get the correct amplitude at the
input of the ADC (between 0 and 5V, with no signal = 2.5V)
To get the best resolution using AD7794, I need to sample at about 4Hz. I
gather that I therefore need an antialiasing filter at around 2Hz or lower,
which would prove very difficult to implement. Needing the resistor values of
the filter to remain low ( because of the Johnson noise), I would need massive
capacitors, very impractical.
Do I really need to filter prior to the AD7794 (audio bandwidth is several
100kHz)? What would happen to the DC part of the signal if there was no filter?
Another method I can think of is to use an audio delta sigma ADC, measure the
whole audio bandwidth and filter in the digital domain (in a DSP), but these
audio ADC usually have poor DC offset characteristics, making it I believe
impossible to get an accurate DC offset measurement.
My last idea is to use an SAR ADC, but I am so far stuck with the resolution of
the SAR (16 bits max usually). With 5V Full scale, a 16 bit step is about 75uV
and I would ideally like a better accuracy. Would averaging in a DSP give me a
much better accuracy, I am not sure. Heavily filtering before the ADC and I am
stuck with the same problem as before.
Do you see other methods to accurately measure the DC offset of such an audio
signal?
Am I making a mistake in my 3 ideas above?
Please bear in mind that the ADC output will be connected to a 400 series Sharc
DSP, so there is some filtering / computing power available.
Your views are greatly appreciated.
Answer
The attached Excel file shows the filter response of the AD7794 for an output
data rate of 4Hz. You should add the standard RC anti-aliasing filter that we
recommend which provides rejection at the modulator sampling frequency. This
anti-aliasing filter along with the AD7794's digital filter should reject the
audio tone and leave the DC signal. I do not think that a filter with a 3dB
point of a few Hz is needed. I have never tried this method before so I am not
guaranteeing it. However, in principle, it should work.