7.3.9.1 Input Driver for the AUX Channel
For applications that use the AUX input channels at high throughput and high input frequency, a driving amplifier with low output impedance is required to meet the ac performance of the internal 16-bit ADC. Some key specifications of the input driving amplifier are discussed below:
- Small-signal bandwidth. The small-signal bandwidth of the input driving amplifier must be much higher than the bandwidth of the AUX input to ensure that there is no attenuation of the input signal resulting from the bandwidth limitation of the amplifier. In a typical data acquisition system, a low cut-off frequency, antialiasing filter is used at the inputs of a high-resolution ADC. The amplifier driving the antialiasing filter must have a low closed-loop output impedance for stability, thus implying a higher gain bandwidth for the amplifier. Higher small-signal bandwidth also minimizes the harmonic distortion at higher input frequencies. In general, Equation 1 can be uses as a basis to calculate the amplifier bandwidth requirements.
Equation 1.
where
- f–3dB is the 3-dB bandwidth of the RC filter
- Distortion. In order to achieve the distortion performance of the AUX channel, the distortion of the input driver (as shown in Equation 2) must be at least 10 dB lower than the specified distortion of the internal ADC.
Equation 2.
- Noise. Careful considerations must be made to select a low-noise, front-end amplifier in order to prevent any degradation in SNR performance of the system. As a rule of thumb, to ensure that the noise performance of the data acquisition system is not limited by the front-end circuit, keep the total noise contribution from the front-end circuit below 20% of the input-referred noise of the ADC. As Equation 3 explains, noise from the input driver circuit is band-limited by the low cut-off frequency of the input antialiasing filter.
Equation 3.
where
- V1 / f_AMP_PP is the peak-to-peak flicker noise
- en_RMS is the amplifier broadband noise density in nV/√Hz
- NG is the noise gain of the front-end circuit, which is equal to 1 in a buffer configuration