REV. 0

AD7741/AD7742

–8–

As can be seen from Table II, the AD7741 has one input range

configuration whereas the AD7742 has unipolar/bipolar as

well as gain options depending on the status of the GAIN

and UNI/

BIP pins.

The transfer function for the AD7741 is shown in Figure 3.

Figure 4 shows the AD7742 transfer function for unipolar input

range configuration while the AD7742 transfer function for

bipolar input range configuration is shown in Figure 5.

OUTPUT

FREQUENCY

0

INPUT

REFIN

Figure 3. AD7741 Transfer Characteristic for Input Range

OUTPUT

FREQUENCY

0

GAIN

+

DIFFERENTIAL

INPUT VOLTAGE

Figure 4. AD7742 Transfer Characteristic for Unipolar

common-mode range must be between +0.5 V and

Table II. AD7741/AD7742 Input Range Selection

UNI/

BIP

GAIN

Gain, G

Part

N/A

N/A

X1

0

AD7741

00X1

AD7742

01X2

AD7742

10X1

0

AD7742

11X2

0

AD7742

OUTPUT

FREQUENCY

DIFFERENTIAL

INPUT VOLTAGE

GAIN

+

GAIN

–

Figure 5. AD7742 Transfer Characteristic for Bipolar

common-mode range must be between +0.5 V and

VFC Modulator

The analog input signal to the AD7741/AD7742 is continu-

ously sampled by a switched capacitor modulator whose sam-

pling rate is set by a master clock input that may be supplied

externally or by a crystal-controlled on-chip clock oscillator.

However, the input signal is buffered on-chip before being ap-

plied to the sampling capacitor of the modulator. This isolates

the sampling capacitor charging currents from the analog input

pins.

This system is a negative feedback loop that tries to keep the net

charge on the integrator capacitor at zero, by balancing charge

The output of the comparator provides the digital input for the

1-bit DAC, so that the system functions as a negative feedback

loop that tries to minimize the difference signal (see Figure 6).

INTEGRATOR

COMPARATOR

+

–

CLK

1-BIT

STREAM

+

INPUT

–

Figure 6. AD7741/AD7742 Modulator Loop