adcdifferentialsingle-ended
I am looking at purchasing a 14-bit ADC chip (http://www.farnell.com/datasheets/1882573.pdf) for a battery monitoring project that I am working on, and am using a differential amplifier measuring voltage across a current shunt and then feeding that into an ADC.
Would this potentially work if I just used the output of the diff amp (1-5V) and into the +in pin and ground into the -in pin? If not, why not?
If breakpoint F is too high and you won't get much image noise rejection. If breakpoint F is too low, you get ISI or phase shift in passband or group delay or attenuation of desired passband.
Any signals above 1/2 Fs create errors as these do not satisfy the Nyquist sampling criteria. If you need additional rolloff on noise above this point, you can choose C to equal the breakpoint in this LPF.
The bandwidth limitation of Op Amps also serves to limit signal harmonic distortion above the 1/2 sample rate. (32MHz here)
Overall you have to decide what your signal range and bandwidth is and what noise rejection you need.
If you had stringent filtering requirements, you might consider a digital filter.
But if not critical just choose C for your breakpoint to be near or below 32MHz by converting the differential equiv cct to a single-ended value for calculations.
Also your choice of a transformer affects your HPF response above DC. YOu might be able to choose an appropriate Video Amp to forego the need for a transformer. But the sharing of grounds often adds to the conversion noise and XFMR's have much high CMMR at 30MHz than Op Amps.
It all depends what what you are sampling and need for accuracy.
Addendum - unrelated to this design. but important for new designers using ADC's.
A good test of your design is use a generator with a time base sweep synchronized with a SA or use a VNA. Or failing that, perform a frequency response test at low levels and high levels and check for harmonic content.
Otherwise with DC response performing a low frequency triangle signal test and using a scope compare A-B with DC coupling or use in X-Y mode with AC coupling. The compare out-in of analog signals should give a difference of +/- LSB at all times through the range. (if conversion lag is small) Often it is not ! so be warned. Analog ground, Vref, missing codes all contribute to this error.
Pseudo differential takes a full positive swing on the +IN input whether it is unipolar or bipolar. If it is bipolar the -IN needs to be fed the nominal dc offset of the signal fed to +IN.
This is an advantage on analogue systems using a mid-rail generator (such as a voltage reference and op-amp buffer). If the voltage reference moves up or down a little, it doesn't affect the ADC code value produced because that offset movement will affect both input signals together.
For a unipolar input where -IN is 0V and a mid-rail generator is used, if the mid-rail generator shifts a little, then that is reflected as a dc error in the converted result.
I tend to use unipolar configurations (even when a mid-rail generator is used) because there appears to me more choice in ADCs AND I always choose really good references PLUS, in a lot of signal processing I do, I don't care about (say) a 10mV offset end-to-end.
For example, the input signal is a single ended sin function with range from 0 to 3V and a DC bias of 1.5V. Should we connect the IN– to 1.5V or to GND?
Best Answer
What you have said above makes no sense - with both input pins grounded, the device won't work. Probably what you want is reassurance that the device can work single-ended (as per your question title. The data sheet quotes this: -
This is on page 16.