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AD7722
(Rev.:Rev0)

16-Bit, 195 kSPS CMOS, Sigma-Delta ADC

Analog Devices 

AD7722

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 the 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 26.

In bipolar mode, the full-scale input range is ± VREF2/2. 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 2s complement

with 1 LSB = 38 µV. The ideal transfer function is shown in

Figure 27.

OUTPUT

CODE

111...111

111...110

111...101

111...100

000...011

000...010

000...001

000...000

0V

VREF2 –1LSB

DIFFERENTIAL INPUT VOLTAGE VIN(+) – VIN(–)

Figure 26. Unipolar Mode Transfer Function

OUTPUT

CODE

011...111

011...110

000...010

000...001

000...000 –VREF2

111...111

111...110

+VREF2/ 2–1LSB

100...001

100...000

0V

DIFFERENTIAL INPUT VOLTAGE VIN(+) – VIN(–)

Figure 27. 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 28. 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.

500Ω

VIN(+) 18

500Ω

VIN(–) 16

CLKIN

ΦA

AD7722

ΦB

2pF

ΦA

2pF

ΦB

AC

GROUND

ΦA ΦB ΦA ΦB

Figure 28. 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 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

AD7722 as shown in Figure 29. 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

R

VIN(+)

C

AD7722

R

VIN(–)

C

Figure 29. 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:

Z IN

=

8

×

109

f CLKIN

kΩ

–14–

REV. 0

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