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EVAL-AD7401EB

Isolated Sigma-Delta Modulator

Analog Devices 

AD7401

DIFFERENTIAL INPUTS

The analog input to the modulator is a switched capacitor

design. The analog signal is converted into charge by highly

linear sampling capacitors. A simplified equivalent circuit

diagram of the analog input is shown in Figure 24. A signal

source driving the analog input must be able to provide the

charge onto the sampling capacitors every half MCLKOUT cycle

and settle to the required accuracy within the next half cycle.

φA

1kΩ

VIN+

φB

2pF

φA

2pF

1kΩ

VIN–

φB

MCLKIN φA φB φA φB

Figure 24. Analog Input Equivalent Circuit

Because the AD7401 samples the differential voltage across its

analog inputs, low noise performance is attained with an input

circuit that provides low common-mode noise at each input.

The amplifiers used to drive the analog inputs play a critical role

in attaining the high performance available from the AD7401.

When a capacitive load is switched onto the output of an op

amp, the amplitude momentarily drops. The op amp tries to

correct the situation and, in the process, hits its slew rate limit.

This nonlinear response, which can cause excessive ringing, can

lead to distortion. To remedy the situation, a low-pass RC filter

can be connected between the amplifier and the input to the

AD7401. The external capacitor at each input aids in supplying

the current spikes created during the sampling process, and the

resistor isolates the op amp from the transient nature of the load.

The recommended circuit configuration for driving

the differential inputs to achieve best performance is shown in

Figure 25. A capacitor between the two input pins sources or

sinks charge to allow most of the charge that is needed by one

input to be effectively supplied by the other input. The series

resistor again isolates any op amp from the current spikes

created during the sampling process. Recommended values for

the resistors and capacitor are 22 Ω and 47 pF, respectively.

R

VIN+

C AD7401

R

VIN–

Figure 25. Differential Input RC Network

Rev. A | Page 14 of 20

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