
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
V
REFIN
A
IN
0.1 F 10 F
1
2
–
5V
R2
R1
R3
R4
R
FS
R
Z
V
IN
3
R1 = R2 = 20k
SEE TABLE II FOR VALUES OF R3, R4, R
, AND R
FS
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
3AGND
V
REFIN
A
IN
V
IN
1
R1
R2
R3
R
FS
R
Z
SEE TABLE III FOR VALUES OF R1, R2, R3, R4, R
, AND R
FS
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
analog input to A
IN
, (see Tables II and III) and adjust R
Z
until the
successive digital output codes flicker between the following codes:
For noninverting, Figure 17
1000 0000 0000
1000 0000 0001
0111 1111 1111
0111 1111 1110
For inverting, Figure 18
Next, adjust full scale by applying a FS–3/2 LSB analog input to
A
IN
, (see Tables II and III) and adjust R
FS
until the successive
digital output codes flicker between the following codes:
For Noninverting, Figure 17
1111 1111 1110
1111 1111 1111
0000 0000 0001
0000 0000 0000
For Inverting, Figure 18
Table II. Resistor and Potentiometer Values Required for
Figure 17
V
IN
Range
V
R3
k
R4
k
R
Z
k
R
FS
k
1/2 LSB
mV
FS/2–3/2 LSB
V
±
2.5
±
5.0
±
10.0
0
20.0
29.8
40.2
19.8
10.0
0.5
0.5
0.5
0.5
1.0
0.5
0.61
1.22
2.44
2.49817
4.99634
9.99268
Table III. Resistor and Potentiometer Values Required for
Figure 18
V
IN
Range
V
R1
k
R2
k
R3
k
R
Z
k
R
FS
k
1/2 LSB
mV
FS/2–3/2 LSB
V
±
2.5
±
5.0
±
10.0
20.0
20.0
20.0
41.2
20.5
10.5
40.2 2
20.0 1
10.2 0.5
1
1
1
0.61
1.22
2.44
2.49817
4.99634
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
V
IN
–
Input Voltage
FS
2
+
–
1LSB
NON-
INVERTING
FIGURE 17
Figure 19. Ideal Input/Output Transfer Characteristics for
Bipolar Input Circuits