AD620B AD [Analog Devices], AD620B Datasheet - Page 13

• Manufacturer Part Number: AD620B
• Description: Low Cost, Low Power Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

Precision V-I Converter
The AD620, along with another op amp and two resistors, makes
a precision current source (Figure 37). The op amp buffers the
reference terminal to maintain good CMR. The output voltage
V
current. This current less only, the input bias current of the op
amp, then flows out to the load.
GAIN SELECTION
The AD620’s gain is resistor programmed by R
cisely, by whatever impedance appears between Pins 1 and 8.
The AD620 is designed to offer accurate gains using 0.1%–1%
resistors. Table II shows required values of R
Note that for G = 1, the R
any arbitrary gain R
To minimize gain error, avoid high parasitic resistance in series
with R
than 10 ppm/ C—for the best performance.
Value of R
REV. E
1% Std Table
X
49.9 k
12.4 k
5.49 k
2.61 k
1.00 k
499
249
100
49.9
V
of the AD620 appears across R1, which converts it to a
V
IN+
IN–
Figure 37. Precision Voltage-to-Current Converter
(Operates on 1.8 mA, 3 V)
G
; to minimize gain drift, R
Table II. Required Values of Gain Resistors
R
G
G
,
I =
L
Calculated
Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
3
8
1
2
V
R1
G
x
can be calculated by using the formula:
=
AD620
[(V
+V
R
–V
IN+
7
4
G
G
S
S
) – (V
R1
pins are unconnected (R
49.4 k
5
IN–
G 1
)] G
G
0.1% Std Table
Value of R
49.3 k
12.4 k
5.49 k
2.61 k
1.01 k
499
249
98.8
49.3
should have a low TC—less
6
AD705
+ V –
R1
G
G
X
,
for various gains.
G
, or more pre-
LOAD
G
100.0
199.4
501.0
Calculated
Gain
2.002
4.984
9.998
19.93
49.91
1,003
= ). For
I
L
–13–
INPUT AND OUTPUT OFFSET VOLTAGE
The low errors of the AD620 are attributed to two sources,
input and output errors. The output error is divided by G when
referred to the input. In practice, the input errors dominate at
high gains and the output errors dominate at low gains. The
total V
REFERENCE TERMINAL
The reference terminal potential defines the zero output voltage,
and is especially useful when the load does not share a precise
ground with the rest of the system. It provides a direct means of
injecting a precise offset to the output, with an allowable range
of 2 V within the supply voltages. Parasitic resistance should be
kept to a minimum for optimum CMR.
INPUT PROTECTION
The AD620 features 400
inputs, and will safely withstand input overloads of up to 15 V
or 60 mA for several hours. This is true for all gains, and power
on and off, which is particularly important since the signal
source and amplifier may be powered separately. For longer
time periods, the current should not exceed 6 mA (I
V
the inputs to the supplies (using a low leakage diode such as an
FD333) will reduce the required resistance, yielding lower
noise.
RF INTERFERENCE
All instrumentation amplifiers can rectify out of band signals,
and when amplifying small signals, these rectified voltages act as
small dc offset errors. The AD620 allows direct access to the
input transistor bases and emitters enabling the user to apply
some first order filtering to unwanted RF signals (Figure 38),
where RC
AD620; C
Pins 1 and 8 and Pins 2 and 3 helps to maintain high CMR.
IN
–IN
+IN
/400 ). For input overloads beyond the supplies, clamping
Total Error RTI = input error + (output error/G)
Total Error RTO = (input error × G) + output error
OS
Figure 38. Circuit to Attenuate RF Interference
R
R
for a given gain is calculated as:
150 pF. Matching the extraneous capacitance at
1/(2 f) and where f
C
C
1
2
3
4
of series thin film resistance at its
R
G
the bandwidth of the
AD620
8
5
7
6
IN