Circuit Design for Low Current [nA] & Low Voltage [uV] Precise/Accurate Measurement.

Question

Hello everyone, 
 Can you please suggest set of 24 Bit high speed ADCs & DAC for low current & voltage measurement. I want precise & fast ADC for measuring current in nano Amperes (nA) & voltage in micro Volt (uV). 
 Necessary speed for data transfer for ADC & reading ADC data. It will be 24 Bit output stream of ADC & 16 Bit input stream for the DAC. 
 VREF of ADC can be from 2.5V to 5V max but 2.5V is preferable.

Schematic is shown above & since ADC is the bottleneck & speed is paramount, therefore please suggest the part numbers which can give me precise and accurate results. 
 I have gone through LTC2485, , AD7760, LTC2380-24, LTC2493, LTC2499, AD7134 these set of ICs already. Please suggest one. I2C interface is preferable. 
 
 Thanks & Regards Arjun

MarkThoren · Answer

Hi Arjun, 
 This is a very open-ended question - how fast do you need to sample? LTC2485 is limited to 15 samples per second in 2x mode, LTC2380-24 is 2 megasamples per second. 
 (I2C devices are almost never "high-speed") 
 How precise (low-noise) do you need the measurements to be? Look at the ADC's RMS input noise spec, choose one that's at least a bit lower noise than your requirements. 
 How accurate (offset error, gain error, linearity error, temperature drift) do you need the measurement to be? You'll have to examine the specs for each of your candidate ADC/DACs. Also - can you correct for any of the errors in software through calibration? 
 But to give you a starting point - the AD7124-8 is a very popular, flexible, general-purpose ADC. 
 The CN0531 is based on the AD5791 20-bit DAC that might suit your needs. 
 Both of these have Linux and no-OS device drivers. 
 If the CN0531 is overkill, have a look at the LTC2641. 
 -Mark

MarkThoren · Answer

Hi Arjun, 
 Again, "high speed" is not a number - is 19.2ksps fast enough for your application? If so, then the AD7124 is a great starting point. You will need to go through the specs - your 0.5nA over 200uA requirement is about 2.5ppm - the AD7124's INL spec is 1ppm typical, so if you truly need 0.5nA over the entire span you will need to do some initial calibration. 
 You would definitely need to calibrate the initial gain error - the best grade LTC6655 has an initial accuracy of about 0.025%, or about 250ppm. But after the initial calibration, temperature and long-term drift are excellent. 
 If 19.2ksps is not fast enough, the AD7768-1 is another option, with sample rates as high as 1024ksps. 
 On the DAC side, the nice thing about the LTC2641(and bipolar LTC2642) is that they have extremely good offset and full-scale error, with a 1lsb maximum linearity error. 
 Hope this helps. 
 -Mark