+15V
14
4
5
10nF
6
7
1nF
13
12
11
10
Channel 4
Software-Zero
9
PGA207
8
2
HI-509
1nF
Input Filter
See Text
–
V
IN
1kΩ
Channel 1
+
V
IN
1kΩ
A/D Converter
ADS7807
15
3
–15V
1
16
74HC574
CK
To Address
Logic
Data Bus
FIGURE 4. Multiplexed-Input Signal Acquisition System.
INPUT COMMON-MODE RANGE
The linear input voltage range of the PGA206 and PGA207
is from approximately 2.3V below the positive supply volt-
age to 1.5V above the negative supply. As a differential
input voltage causes output voltage to increase, however, the
linear input range is limited by the output voltage swing of
amplifiers A
1
and A
2
. So the linear common-mode input
range is related to the output voltage of the complete
amplifier. This behavior also depends on supply voltage—
see performance curves “Input Common-Mode Range vs
Output Voltage”.
Input overload can produce an output voltage that appears
normal. For example, if an input overload condition drives
both input amplifiers to their positive output swing limit, the
difference voltage measured by the output amplifier will be
near zero. The output of the PGA206 or PGA207 will be
near 0V even though both inputs are overloaded. This
condition can be detected by sensing the voltage on the V
01
and V
02
pins to determine whether they are within their
linear operating range.
INPUT PROTECTION
The inputs of the PGA206 and PGA207 are individually
protected for voltages up to
±40V.
For example, a condition
of -40V on one input and +40V on the other input will not
cause damage. Internal circuitry on each input provides low
series impedance under normal signal conditions. If the input
is overloaded, the protection circuitry limits the input current
to a safe value. The typical performance curve “Input Over-
load V/I Characteristic” shows this behavior. The inputs are
protected even if no power supply voltage is applied.
®
MULTIPLEXED INPUTS
The PGA206 and PGA207 are ideally suited for multiple
channel data acquisition. Figure 4 shows a typical applica-
tion with an analog multiplexer used to connect one of four
differential input signals to a single PGA207.
Careful circuit layout will help preserve accuracy of multi-
plexed signals. Run the inverting and non-inverting connec-
tions of each channel parallel to each other over a ground
plane, or directly adjacent on top and bottom of the circuit
board. Grounded guard traces between channels help reduce
stray signal pick-up.
Multiplexed signals from high impedance sources require
special care. As inputs are switched by the multiplexer,
charge can be injected into the source, disturbing the input
signal. Since many such sources involve slow signals, a
simple R/C filter at the input can be used to dramatically
reduce this effect. The arrangement shown filters both the
differential signal and common-mode noise.
9
PGA206/207
BURR-BROWN [ BURR-BROWN CORPORATION ]
