In a typical photodiode analog front-end application, a AD820 is used as a transimpedance amplifier and a second one as inverter and ADC driver. The 3-dB bandwidth is about 7 kHz. The ADC used to have 12-bit resolution, but it is going to be replaced by a 16-bit SAR.

However, the applications part of the AD820 datasheet reads : "12-bit to 14-bit data acquisition systems". Does its THD of -93 dB absolutely rule out a 16-bit ADC?

Hi Maurizio,

The parameter that you to need consider is the ENOB which is the effective number of bits. This parameter depends on THD (total harmonic distortion) and SNR (signal-to-noise ratio) which forms the SINAD (signal-to-noise and distortion ratio). This mini tutorial might be able to help you understand those parameters.

If we assume the best case SINAD using the AD820, and assume that THD is the only source of error excluding the noise. The THD of AD820 is 93 dB.

Using the formula for computing the ENOB,

ENOB = (SINAD - 1.76) / 6.02 = (93 - 1.76) / 6.02 = 15 bits

So based on the calculation, the best case ENOB would be 15 bits. And of course, if we include and consider the effect of the total noise and distortion of the AD820 and your ADC, more likely your ENOB will be less than 15 bits.

I hope this helps.

Regards,

Jay