Datasheet

PGA870

ADC

IN-

IN+

PGA870

SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011

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Differential phase shift is the difference in the phase response between the two branches of the differential filter

signal path. Differential phase shift in the filter as a result of mismatched components caused by nominal

tolerance can severely degrade the even-order distortion of the amplifier-ADC chain. This condition has the same

effect as mismatched path lengths for the two differential traces, and causes more phase shift in one path than

the other. Ideally, the phase response over frequency through the two sides of a differential signal path are

identical, such that even-order harmonics remain optimally out of phase and cancel when the signal is taken

differentially. However, if one side has more phase shift than the other, then the even-order harmonic

cancellation is not as effective.

Single-order RC filters cause very little differential phase shift with nominal tolerances of 5% or less, but

higher-order LC filters are very sensitive to component mismatch. For instance, a third-order Butterworth

bandpass filter with 100-MHz center frequency and 20-MHz bandwidth shows up to 20° differential phase

imbalance in a Spice Monte Carlo analysis with 2% component tolerances. Therefore, while a prototype may

work, production variance is unacceptable. In ac-coupled applications that require second- and higher-order

filters between the PGA870 and ADC, a transformer or balun is recommended at the ADC input to restore the

phase balance. For dc-coupled applications where a transformer or balun at the ADC input cannot be used, it is

recommended to use first- or second-order filters to minimize the effects of differential phase shift as a result of

component tolerance.

ADC Input Common-Mode Voltage Considerations: AC-Coupled Input

The input common-mode voltage range of the ADC must be respected for proper operation. In an ac-coupled

application between the amplifier and the ADC, the input common-mode voltage bias of the ADC is

accomplished in different ways depending on the specific ADC. Some ADCs use internal bias networks, and the

analog inputs are automatically biased to the required input common-mode voltage if the inputs are ac-coupled

with capacitors (or if the filter between the amplifier and ADC is a bandpass filter). Other ADCs supply the

required input common-mode voltage as a reference voltage output at a CM pin. With these types of ADCs, the

ac-coupled input signal can be re-biased to the input common-mode voltage by connecting resistors from each

input to the CM output of the ADC, as shown in Figure 49. However, the signal is attenuated because of the

voltage divider created by R

and R

Figure 49. Biasing AC-Coupled ADC Inputs with the ADC CM Output

The signal can be re-biased when ac coupling, and therefore the output common-mode voltage of the amplifier is

a don’t care for the ADC.

Product Folder Link(s): PGA870