24 bit ADC without op amp !

[ranaya]

hi every1.....

this question is about smart sensor applications.
can 24 bit ADC directly interface to a sensor which outputs nearly 1 - 10 mV in range ? do i have to amplify it before ADC ? if not, does it much affect to the accuracy ?

Thnx

 

[kemiyun]

I'm not experienced with signal conditioning circuits for ADC's but here is what just popped into my head. Say you have 3.3V supply and ADC can perfectly samples this dynamic range. If you do the calculation the corresponding voltage for 1 LSB is 1.96695328*10^-7 Volts. And your 9 mV range contains 9mV/1.96695328*10^-7. And if you take the logarithm of log(45756.0436)/log(2) this. Voila! you will have a 15.5 bits of resolution. Considering true 24 bit ADC's are quite an asset it will be wasteful to use them this way. If the area is your main concern some ADC's may have their signal conditioning circuit integrated you might want to consider them.

 

[FvM]

As kemiyun already mentioned, many low speed 24-Bit ADC have built-in signal conditioning, involving a PGA. In so far you get more resolution than calculated. But all recent high resolution ADCs are CMOS mixed signal ICs, and their amplifiers don't have mind blowing noise specs. Even a so called ADC with ultra low noise PGA like CS5532 has a considerable degradation of noise free resolution at higher gains. You should at least compare the achievable noise performance with the results using an external amplifier like OPA227.

 

[keith1200rs]

You need to also consider the impedance of you source signal and the input characteristics of the ADC. A high impedance source with an ADC which requires a low impedance drive will not work well. However, I think there are some ADCs with built in programmable instrumentation amplifiers, so that could be a possibility.

Keith