4 AAF design trade-offs

The parameters in this interface circuit are very interactive; therefore, it is almost impossible to optimize the circuit for the primary specifications (bandwidth, bandwidth flatness, SNR, SFDR and gain) without small trade-offs. However, you can minimize bandwidth peaking, which often occurs at the tail end of the bandwidth response, by varying R_A, R_KB, or both; either can have a net positive or net negative affect on AAF bandwidth performance.

Notice in Figure 3 how the pass-band peaking is enhanced or flattened as the value of the FDA’s output series resistance (R_A) changes (the blue dashed curves). As the value of this resistance decreases, there is more signal peaking, and the amplifier can drive the signal less to fill the ADC’s full-scale input range at the cost of the pass-band flatness response near the edge of the AAF frequency response.

The value of R_A could also affect SNR performance. Smaller values, while enhancing bandwidth peaking, tend to decrease the SNR because of the increased bandwidth and unwanted noise.

It’s also important to select the R_KB series resistor on the ADC inputs to minimize distortion caused by any residual charge injection from the internal sampling capacitor within the ADC. However, increasing this resistor also tends to enhance or reduce bandwidth peaking as well, depending on the filter topology.

When optimizing for the AAF’s rolloff frequency, varying C_AAF2 by a small amount allows you to correct for optimal frequency coverage for the application.

Normally, determining the value of the ADC input termination resistor, R_TADC, makes the net ADC input impedance look near typical of most amplifier characteristic load (R_L) values. Selecting too high or too low a value for R_TADC can have an adverse effect on the amplifier’s linearity, which will then be reflected in the overall SFDR signal-chain lineup.