18.104.22.168 Input Amplifier Selection
Selection criteria for the input amplifiers is highly dependent on the input signal type, as well as the performance goals, of the data acquisition system. Some key amplifier specifications to consider when selecting an appropriate amplifier to drive the inputs of the ADC are:
- Small-signal bandwidth. Select the small-signal bandwidth of the input amplifiers to be as high as possible after meeting the power budget of the system. Higher bandwidth reduces the closed-loop output impedance of the amplifier, thus allowing the amplifier to more easily drive the ADC sample-and-hold capacitor and the RC filter (Charge-Kickback Filter) at the inputs of the ADC. Higher bandwidth amplifiers offer faster settling times while driving the capacitive load of the charge-kickback filter, thus reducing harmonic distortion at higher input frequencies. In order to maintain the overall stability of the input driver circuit, select the amplifier with a unity gain bandwidth (UGB) as described in Equation 16:
- Distortion. Both the ADC and the input driver introduce distortion in a data acquisition block. To make sure that the distortion performance of the data acquisition system is not limited by the front-end circuit, the distortion of the input driver must be at least 10 dB less than the distortion of the ADC, as shown in Equation 17.
- Noise. Noise contribution of the front-end amplifiers must be as low as possible to prevent any degradation in SNR performance of the system. Generally, to make sure that the noise performance of the data acquisition system is not limited by the front-end circuit, the total noise contribution from the front-end circuit must be kept below 20% of the input-referred noise of the ADC. Noise from the input driver circuit is band-limited by designing a low cutoff frequency, charge-kickback filter, as explained in Equation 18.
- V1 / f_AMP_PP is the peak-to-peak flicker noise in µV
- en_RMS is the amplifier broadband noise density in nV/√Hz
- f–3dB is the 3-dB bandwidth of the charge-kickback filter
- NG is the noise gain of the front-end circuit that is equal to 1 in a buffer configuration
- Settling Time. For dc signals with fast transients that are common in a multiplexed application, the input signal must settle within an 18-bit accuracy at the device inputs during the acquisition time window. This condition is critical to maintain the overall linearity performance of the ADC. Typically, amplifier data sheets specify the output settling performance only up to 0.1% to 0.001%, which may not be sufficient for the desired 18-bit accuracy. Therefore, always verify the settling behavior of the input driver by TINA-TI SPICE simulations before selecting the amplifier.