Manual output offset trim circuit.
V+
VO1 V+
13
Resistors can be substituted
1
for REF200. Power supply
rejection will be degraded.
100µA
1/2 REF200
PGA206
PGA207
4
Over-Voltage
Protection
–
VIN
A1
100Ω
100Ω
12
11
10kΩ
10kΩ
R2
10kΩ
16
A1
A0
Digitally Selected
Feedback Network
A3
VO
15
14
100µA
Digital
1/2 REF200
Ground
VREF
10
V–
A2
5
Over-Voltage
Protection
+
10kΩ
10kΩ
VIN
Offset control with
OPA131
digital/analog converter
D/A
6
7
9
8
VO2
V–
Optional Input Offset Adjustment
R1
200kΩ
(100kΩ to 500kΩ)
V+
FIGURE 2. Optional Offset Voltage Trim Circuits.
R1 adjusts the offset of the input amplifiers. Output stage
offset is adjusted with R2. A buffer op amp is required in
the output offset adjustment circuit, as shown, to assure
that the Ref pin is driven by a low source impedance. To
adjust for low offset voltage in all gains, first adjust the
input stage offset in the highest gain. Then adjust the
output stage offset (R2) in G = 1. Iterate the adjustments for
lowest offset in all gains.
Microphone,
Hydrophone
etc.
PGA
47kΩ
47kΩ
Offset can also be adjusted under processor control with a
D/A converter as shown in Figure 2. The D/A’s output
voltage can be reduced with a resistor divider for better
adjustment resolution, but an op amp buffer following the
divider is required to provide a low source impedance to
the ref terminal. A different offset value is required for
each amplifier gain.
Thermocouple
PGA
10kΩ
INPUT BIAS CURRENT RETURN PATH
The FET inputs of the PGA206 and PGA207 provide ex-
tremely high input impedance. Still, a path must be provided
for the bias current of each input. Figure 3 shows provisions
for an input bias current path. Without a bias current return
path, the inputs will float to a potential which exceeds the
linear input voltage range and the input amplifiers will
saturate.
PGA
VR
Center-tap provides
bias current return.
If the differential source resistance is low, a bias current
return path can be connected to only one input (see thermo-
couple example in Figure 3). With higher source impedance,
using two resistors provides a balanced input with possible
advantages of lower input offset voltage due to bias current
and better common-mode rejection.
Bridge
PGA
Bias current return
inherrently provided by source.
Many sources or sensors inherently provide a path for input
bias current (e.g. the bridge sensor shown in Figure 3).
These applications do not require additional resistor(s) for
proper operation.
FIGURE 3. Providing an Input Bias Current Path.
®
8
PGA206/207
BB [ BURR-BROWN CORPORATION ]
