AD526 Analog Devices, Inc., AD526 Datasheet - Page 11

• Manufacturer Part Number: AD526
• Description: Software Programmable Gain Amplifier
• Manufacturer: Analog Devices, Inc.

REV. D
OFFSET NULLING
Input voltage offset nulling of the AD526 is best accomplished
at a gain of 16, since the referred-to-input (RTI) offset is ampli-
fied the most at this gain and therefore is most easily trimmed.
The resulting trimmed value of RTI voltage offset typically
varies less than 3 V across all gain ranges.
Note that the low input current of the AD526 minimizes RTI
voltage offsets due to source resistance.
OUTPUT CURRENT BOOSTER
The AD526 is rated for a full 10 V output voltage swing into
2 k . In some applications, the need exists to drive more cur-
rent into heavier loads. As shown in Figure 40, a high current
booster may be connected “inside the loop” of the SPGA to
provide the required current boost without significantly degrad-
ing overall performance. Nonlinearities, offset and gain inaccu-
racies of the buffer are minimized by the loop gain of the
AD526 output amplifier.
V
IN
V
IN
16
A1
1
AD526
16
LOGIC AND LATCHES
A0
15
2
16
A1
1
Figure 39. Offset Voltage Null Circuit
Figure 40. Current Output Boosting
GAIN NETWORK
AD526
8
16
14
CS CLK
LOGIC AND LATCHES
3
A0
15
2
4
GAIN NETWORK
20k
8
13
4
14
CS CLK
3
2
4
12
A2
5
13
1
4
2
11
B
6
12
A2
5
1
+
–
–V
+V
10
7
11
B
0.1 F
0.1 F
S
6
S
–
+
+V
–V
9
8
10
7
S
S
OUT
FORCE
OUT
SENSE
9
8
0.1 F
0.1 F
OUT
FORCE
OUT
SENSE
HOS-100
0.01 F
0.01 F
V
OUT
R
L
–11–
CASCADED OPERATION
A cascade of two AD526s can be used to achieve binarily
weighted gains from 1 to 256. If gains from 1 to 128 are needed,
no additional components are required. This is accomplished by
using the B pin as shown in Figure 38. When the B pin is low,
the AD526 is held in a unity gain stage independent of the other
gain code values.
OFFSET NULLING WITH A D/A CONVERTER
Figure 41 shows the AD526 with offset nulling accomplished
with an 8-bit D/A converter (AD7524) circuit instead of the
potentiometer shown in Figure 39. The calibration procedure is
the same as before except that instead of adjusting the potenti-
ometer, the D/A converter corrects for the offset error. This
calibration circuit has a number of benefits in addition to elimi-
nating the trimpot. The most significant benefit is that calibra-
tion can be under the control of a microprocessor and therefore
can be implemented as part of an autocalibration scheme. Sec-
ondly, dip switches or RAM can be used to hold the 8-bit word
after its value has been determined. In Figure 42 the offset null
sensitivity, at a gain of 16, is 80 V per LSB of adjustment,
which guarantees dc accuracy to the 16-bit performance level.
MSB
LSB
WR
CS
+V
S
Figure 41. Offset Nulling Using a DAC
AD7524
V
REF
V
AD581 OR
IN
AD587
GND
+10V
16
A1
1
AD526
16
LOGIC AND LATCHES
15
A0
2
OUT 2
3.3M
GAIN NETWORK
1k
OUT 1
8
14
CS CLK
3
4
ALL BYPASS CAPACITORS ARE 0.1 F
13
4
10 F
7.5M
2
12
A2
5
1
AD548
–
+
11
B
6
+V
–V
0.01 F
S
+
S
–
0.01 F
+V
–V
10
7
S
S
9
8
AD526
0.1 F
0.1 F
OUT
FORCE
OUT
SENSE
V
OUT