AD7837/AD7847

REV. C

–9–

APPLICATIONS

PROGRAMMABLE GAIN AMPLIFIER (PGA)

make the AD7837 ideal as a programmable gain amplifier. In this

application, the DAC functions as a programmable resistor in the

amplifier feedback loop. This type of configuration is shown

in Figure 17 and is suitable for ac gain control. The circuit con-

sists of two PGAs in series. Use of a dual configuration provides

greater accuracy over a wider dynamic range than a single PGA

solution. The overall system gain is the product of the individual

gain stages. The effective gains for each stage are controlled by

the DAC codes. As the code decreases, the effective DAC

resistance increases, and so the gain also increases.

DAC B

AGNDA

AD7837

AGNDB

DAC A

Figure 17. Dual PGA Circuit

The transfer function is given by

=

×

(1)

by the digital input code:

=

N

(2)

N = DAC input code in decimal.

The transfer function in (1) thus simplifies to

=

×

(3)

input code in decimal.

code is not allowed as it results in an open loop amplifier

should be chosen to be equal to or as close as possible to each

other to achieve the required gain.

BIPOLAR OPERATION

(4-QUADRANT MULTIPLICATION)

Figure 16 shows the AD7837/AD7847 connected for bipolar

operation. The coding is offset binary as shown in Table IV.

plication. To maintain the gain error specifications, resistors R1,

R2 and R3 should be ratio matched to 0.01%. Note that on the

DAC A

AGNDA

DGND

*

AD7837

AD7847

*INTERNALLY

CONNECTED

ON AD7847

R3

10k

R1

20k

AD711

R2

20k

Figure 16. Bipolar Offset Binary Operation

Table IV. Bipolar Code Table

DAC Latch Contents

MSB

LSB

1111 1111 1111

+V

IN

×

2047

2048









1000 0000 0001

+V

IN

×

1

2048









1000 0000 0000

0 V

0111 1111 1111

×

1

2048









0000 0000 0000

×

2048

2048









= –V

IN

Note 1 LSB =

V

IN

2048

.