REV. B

ADC912A

–10–

BIPOLAR ANALOG INPUT OPERATION

Bipolar analog input operation is achieved with an external

amplifier providing an analog offset. Figures 17 and 18 show

two circuit topologies that result in different digital-output cod-

ing. In Figure 17, offset binary coding is produced when the

external amplifier is connected in the inverting mode. Figure 19

shows the ideal transfer characteristics for both the inverting

and noninverting configurations given in Figures 17 and 18.

AGND

0.1 F 10 F

1

2

–5V

R2

R1

R3

R4

3

R1 = R2 = 20k

A1: OP27 LOWEST NOISE, OP42 BEST BANDWIDTH

*EXTRA PINS OMITTED FOR CLARITY

ADC912A*

A1

Figure 17. Noninverting Bipolar Analog Input Operation

The scaling resistors chosen in bipolar input applications should

be from the same manufacturer to obtain good resistor tracking

performance over temperature. When potentiometers are used

for absolute adjustment, 0.1% tolerance resistors should still be

used as shown in Figures 17 and 18 to minimize temperature

coefficient errors.

–5V

0.1 F 10 F

2

3 AGND

1

R1

R2

R3

A1: OP27 LOWEST NOISE, OP42 BEST BANDWIDTH

*EXTRA PINS OMITTED FOR CLARITY

ADC912A*

+

A1

Figure 18. Inverting Bipolar Analog Input

Calibration of the bipolar analog input circuits (Figures 17 and

18) should begin with zero adjustment first. Apply a +1/2 LSB

successive digital output codes flicker between the following codes:

For noninverting, Figure 17

1000 0000 0000

1000 0000 0001

For inverting, Figure 18

0111 1111 1111

0111 1111 1110

Next, adjust full scale by applying a FS–3/2 LSB analog input to

digital output codes flicker between the following codes:

For Noninverting, Figure 17

1111 1111 1110

1111 1111 1111

For Inverting, Figure 18

0000 0000 0001

0000 0000 0000

Table II. Resistor and Potentiometer Values Required for

Figure 17

R3

R4

1/2 LSB

FS/2–3/2 LSB

Vk

k

k

k

mV

V

±2.5

0

40.2

0.5

0.5

0.61

2.49817

±5.0

20.0

19.8

0.5

1.0

1.22

4.99634

±10.0

29.8

10.0

0.5

0.5

2.44

9.99268

Table III. Resistor and Potentiometer Values Required for

Figure 18

R1

R2

R3

1/2 LSB

FS/2–3/2 LSB

Vk

k

k

k

k

mV

V

±2.5

20.0 41.2 40.2 2

1

0.61

2.49817

±5.0

20.0 20.5 20.0 1

1

1.22

4.99634

±10.0

20.0 10.5 10.2 0.5

1

2.44

9.99268

111...110

100...000

111...111

100...001

011...111

011...110

000...001

000...000

DIGITAL OUTPUT

0V

FS

2

–

FS

2

+

INVERTING

FIGURE 18

FS

2

+

– 1LSB

NON-

INVERTING

FIGURE 17

Figure 19. Ideal Input/Output Transfer Characteristics for

Bipolar Input Circuits