AD7722 datasheet(15/24 Pages) AD

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REV. B

AD7722

–15–

APPLYING THE AD7722

Analog Input Range

The AD7722 uses differential inputs to provide common-mode

noise rejection (i.e., the converted result will correspond to the

differential voltage between the two inputs). The absolute voltage

on both inputs must lie between AGND and AVDD.

In unipolar mode, the full-scale analog input range

(VIN(+) – VIN(–)) is 0 V to VREF2. The output code is straight

binary in the unipolar mode with 1 LSB = 38

µV. The ideal transfer

function is shown in Figure 11.

In bipolar mode, the full-scale input range is

±V

REF2/2. The

bipolar mode allows complementary input signals. As another

example, in bipolar mode, VIN(–) can be connected to a dc bias

voltage to allow a single-ended input on VIN(+) equal to VBIAS

±VREF2/2. In bipolar mode, the output code is twos complement

with 1 LSB = 38

µV. The ideal transfer function is shown in

Figure 12.

111...111

111...110

111...101

111...100

000...011

000...010

000...001

000...000

OUTPUT

CODE

VREF2 –1LSB

DIFFERENTIAL INPUT VOLTAGE V IN(+) – V IN(–)

Figure 11. Unipolar Mode Transfer Function

111...111

111...110

100...001

100...000

000...010

000...001

000...000

OUTPUT

CODE

+VREF2/2 – 1LSB

DIFFERENTIAL INPUT VOLTAGE V IN(+) – V IN(–)

–VREF2

011...111

011...110

Figure 12. Bipolar Mode Transfer Function

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 13. A signal source driving

the analog input must be able to provide the charge onto the

sampling capacitors every half CLKIN cycle and settle to the

required accuracy within the next half cycle.

ΦA

ΦB

ΦA

ΦB

2pF

GROUND

500

ΦA ΦB ΦA ΦB

CLKIN

VIN(+)

VIN(–)

AD7722

500

Figure 13. Analog Input Equivalent Circuit

Since the AD7722 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 AD7722.

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

the amplitude will momentarily drop. The op amp will try to

correct the situation and, in the process, will hit 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

AD7722 as shown in Figure 14. The external capacitor at each

input aids in supplying the current spikes created during the

sampling process. The resistor in this diagram, as well as creating

the pole for the antialiasing, isolates the op amp from the transient

nature of the load.

ANALOG

INPUT

AD7722

VIN(+)

VIN(–)

Figure 14. Simple RC Antialiasing Circuit

The differential input impedance of the AD7722 switched capacitor

input varies as a function of the CLKIN frequency, given by the

equation

CLKIN

Ω

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