### Q

In AD7712 data sheet, pages 11 and 12, noise is specified as total rms value. Iwould like to know noise spectral density ( in particular for a 1kHz notch

frequency selection and in a 1 mHz to 1 Hz bandwidth ). Have you got some

specifications or estimations even if not garanted ?

If noise for a 10Hz notch frequency selection ( -3dB at 2.62 Hz ) is specified

at 1 uV ( in datasheet for a x1 gain ) can we assume a 1uV/sqrt(2.62Hz)=0.6

uV/sqrt(Hz) noise spectral density at low frequecies even if we are using a 1kHz

notch frequency selection ? Is noise increasing with a 1/f law at low frequncies

and how much would the "corner frquency" ?

### A

There is chopping internally on the front end integrator of the AD7712 whichwill eliminate any 1/f noise from the integrator. I checked with our designers

and although we have not explicitly looked at noise spectral density at high

update rates, our low update rate tests would indicate that the noise at low

frequency is dominated only by thermal noise and therefore essentially flat.

The calculation you propose of dividing the rms noise by the square root of the

bandwidth assumes a “brick wall” bandwidth i.e. an infinitely sharp cutoff and

transition band. To get a close approximation of the noise spectral density you

will have to calculate the equivalent brick wall bandwidth of a (sin(x)/x)^3

filter.

The Noise spectral density at low frequency will not change with update date

rate but it will change with gain. This is because we use a mixture of

different size sample capacitors and different integration periods in order to

realize the different gain settings.

Also note: at low frequencies noise is dominated by the thermal (white) noise

of the silicon. At high frequencies the noise is dominated by the quantisation

noise. The cross over point where quantisation noise starts to dominate is

between 100 and 300Hz. At an update rate of 1kHz you are going to see a lot of

high frequency quantisation noise.

I presume from your query that you propose to do some external post filtering.

Post filtering will allow you to customise the response and settling time of

the decimating filter. However, any digital filter is periodic about the

sampling frequency and offers no attenuation at integer multiples of the

sampling frequency.