Recently I wrote an article which appeared in the Volume 47 - February 2013 edition of Analog Dialogue. This article outlines a method of estimating the noise introduced into an ADC signal chain by an amplifier or driver. Amplifiers or drivers can be used to achieve one or a combination of the following objectives:
Provide gain to the input signal to increase the ADC resolution.
Buffer or transform the impedance between the input source and the ADC.
Convert a single-ended input signal to a differential output signal.
In doing so, the amplifier introduces noise into the ADC inputs thereby affecting its performance (SNR). In this article, I explain how to interpret the datasheet of an amplifier and that of an ADC and using some basic math, arrive at an estimated SNR when a particular amplifier-ADC combination is chosen.
The ADC chosen for this experiment is the AD9268-125 and the Variable Gain Amplifier (VGA) is the AD8352. Using the datasheet numbers for the Noise Spectral Density (NSD) of the amplifier (20nV/√Hz into a 150Ω load), I can prove that the performance of the system that uses the AD8352 to drive the AD9268 will be dominated by the amplifier noise. This means that using the AD8352 to drive the AD9268 will result in an ENOB of about 11.8bits.
This same technique can be used to estimate the performance of any amplifier – ADC combination thereby helping the customer make a more informed decision on the amplifier selection for a particular ADC.