PGA203AG BURR-BROWN [Burr-Brown Corporation], PGA203AG Datasheet - Page 9

• Manufacturer Part Number: PGA203AG
• Description: Digitally Controlled Programmable-Gain INSTRUMENTATION AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

INPUT CHARACTERISTICS
Because the PGA202/203 have FET inputs, the bias currents
drawn through input source resistors have a negligible effect
on DC accuracy. The picoamp currents produce no more
than microvolts through megohm sources. The inputs are
also internally diode clamped to the supplies. Thus, input
filtering and input series protection are easily achievable.
A return path for the input bias currents must always be
provided to prevent the charging of any stray capacitance.
Otherwise, the amplifier could wander and saturate. A 1M
to 10M
floating sources such as thermocouples and AC-coupled
inputs (see Applications Section, Figures 8 and 9.)
DYNAMIC PERFORMANCE
The PGA202 and the PGA203 are fast-settling FET input
programmable gain instrumentation amplifiers. Careful at-
tention to minimize stray capacitance is necessary to achieve
specified performance. High source resistance will interact
with the input capacitance to reduce speed and overall
bandwidth. Also, to maintain stability, avoid capacitance
from the output to the input or the offset adjust pins.
Applications with balanced source impedance will provide
the best performance. In some applications, mismatched
source impedances may be required. If the impedance in the
negative input exceeds that in the positive input, stray
capacitance from the output will create a net negative feed-
back and improve the stability of the circuit. If, however, the
impedance in the positive input is greater, then the feedback
due to stray capacitance will be positive and instability may
result. The degree of positive feedback will, of course,
depend on the source impedance imbalance as well as the
board layout and the operating gain. The addition of a small
bypass capacitor of about 5 to 50pF directly across the input
terminals of the PGIA will generally eliminate any instabil-
ity arising from these stray capacitances. CMR errors due to
the source imbalance will also be reduced by the addition of
this capacitor.
The PGA202 and the PGA203 are designed for fast settling
in response to changes in either the input voltage or the gain.
The bandwidth and the settling times are mostly determined
by the output stage and are therefore independent of gain,
except at the highest gain of the PGA202 where other factors
in the input stage begin to dominate.
APPLICATIONS
In addition to general purpose applications, the PGA202/203
are designed to handle two important and demanding classes
of applications: inputs with high source impedances, and
rapid scanning data acquisition systems requiring fast set-
tling time. Because the user has access to output sense and
output common pins, current sources can also be constructed
with a minimum of external components. Some basic appli-
cation circuits are shown in Figures 6 through 12.
resistor from the input to common will return
9
FIGURE 6. Isolated Programmable Gain Instrumentation
FIGURE 7. Auto Gain Ranging.
FIGURE 8. AC-Coupled Differential Amplifier for
OSC
V
V
IN
IN
8
7
8
7
Up/Down
Counter
–
+
+
–
PGA203
PGA202
2-Bit
1
Amplifier.
1
1µF
1µF
1M
Frequencies Above 0.16Hz.
14
14
2
2
+5
11
4
Down
Up
11
4
1M
12
12
R
PGA202/203
L
Comparator
8
7
Dual
ISO
+
–
PGA202
–
–
+
+
1
Coupler
Digital
Opto-
14
2
Gain Control
102
11
4
12
R
10k
R
1k
V
V
1
2
OUT
V
OUT
D
OUT
IN
V
10V
REF
®