6

Bipolar (Offset Binary) Operation

The circuit configuration for operating the AD7520 in the

bipolar mode is given in Figure 9. Similar circuits can be

used for AD7521. Using offset binary digital input codes

and positive and negative reference voltage values,

4-Quadrant multiplication can be realized. The “Digital

Input Code/Analog Output Value” table for bipolar mode is

given in Table 2.

A “Logic 1” input at any digital input forces the corresponding

ladder switch to steer the bit current to IOUT1 bus. A “Logic 0”

input forces the bit current to IOUT2 bus. For any code the

IOUT1 and IOUT2 bus currents are complements of one

another. The current amplifier at IOUT2 changes the polarity

of IOUT2 current and the transconductance amplifier at

IOUT1 output sums the two currents. This configuration

doubles the output range. The difference current resulting

at zero offset binary code, (MSB = “Logic 1”, all other

bits = “Logic 0”), is corrected by using an external resistor,

(10MW), from VREF to IOUT2 .

Offset Adjustment

2. Connect all digital inputs to “Logic 1”.

4. Connect MSB (Bit 1) to “Logic 1” and all other bits to

“Logic 0”.

Gain Adjustment

1. Connect all digital inputs to V+.

AD7520, and N = 10 for AD7521.).

TABLE 2. BlPOLAR (OFFSET BINARY) CODE TABLE

DIGITAL INPUT

ANALOG OUTPUT

1111111111

1000000001

1000000000

0

0111111111

0000000001

0000000000

NOTES:

2. N = 8 for 7520

N = 10 for 7521.

15

16

1

5

4

13 3

2

AD7520

BIT 1

BIT 10

14

+15V

6

-

+

6

-

+

(MSB)

(LSB)

R1 10K

0.01%

R2 10K

0.01%

R3

10M

Ω

FIGURE 9. BIPOLAR OPERATION (4-QUADRANT

MULTIPLICATION)

AD7520, AD7521