feeding ADC with too big input swing

What part?

A lot of the video adc's have diff inputs that want to center on a common mode like you name, +1.25 or so. Some provide a Vref of that voltage. You can usually drive them single-ended, with the other input held at some stiff voltage, like 1.25. I say "stiff" because sometimes signal current will sneak out of the undriven input.

The price of going single-ended is usually a slightly worse s/n and some additional harmonic distortion.

ADCs are noisy because they have a lot of stuff in the signal paths, not to mention all the digital junk on the same chip. Opamps are much simpler, and usually only two transistors matter to the s/n. And opamps don't have quantization noise!

John

It depends. AC or DC coupling, high or low frequency, source/drive/referrence input impedance, how many channels, noise requirements, etc.

For the low power low frequency, the minimum solution is something like this:

Getting away from the ground bounce and using the full swing of the ADC.

It depends. A good ADC can actually be better then a good opamp.

No. The whole thing can be fully differential.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Hi, thanks for the reply!

Its definitely low-power (a monochrome video-signal), but the ADC (LTC2202) is sampling at 10 mhz and the signal is around half of that. One channel, DC-drive. Noise preferably in the ~100 uV range (I know this is larger than the 16-bit ADC LSB). My first idea was to simply use the LT1994 which is in one of Linears recommended input drives for the LTC2202, when converting a single-ended input. It's just that my input is 0-5V which requires some dampening.

I guess by separating the grounds of the single-ended input-device (a sensor) with the ADC-system you could have a differential input, if that is what you mean.

Best regards, Bjorn W

Fully differential opamps can certainly be used; however they are pricey, power hungry, not exactly very low noise (1/F at low frequencies), and the assortment is quite limited. You can also take a look at OPA1632 and AD8137.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant