AD621 Analog Devices, Inc., AD621 Datasheet - Page 13

• Manufacturer Part Number: AD621
• Description: Low Drift, Low Power Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

Precision V-I Converter
The AD621 along with another op amp and two resistors make
a precision current source (Figure 9). The op amp buffers the
reference terminal to maintain good CMR. The output voltage
V
rent. This current less only the input bias current of the op amp
then flows out to the load.
INPUT AND OUTPUT OFFSET VOLTAGE
The AD621 is fully specified for total input errors at gains of 10
and 100. That is, effects of all error sources within the AD621
are properly included in the guaranteed input error specs, elimi-
nating the need for separate error calculation.
REFERENCE TERMINAL
Although usually grounded, the reference terminal may be used
to offset the output of the AD621. This is useful when the load
is “floating” or does not share a ground with the rest of the system.
It also provides a direct means of injecting a precise offset.
Another benefit of having a reference terminal is that it can be
quite effective in eliminating ground loops and noise in a circuit
or system.
X
of the AD621 appears across R1 which converts it to a cur-
Total Error RTO = (Input Error × G) + Output Error
Total Error RTI = Input Error + (Output Error/G)
V
V
IN+
IN–
I
L
GAIN = 10 OR 100
=
R1
V
X
V
V
=
OL
OL
(V
IN+
R
R
P
P
AD621
) – (V
+V
–V
R1
S
S
±
IN–
) G
AD621
–V
+V
S
S
AD705
+V
R1
X–
V
OUT
LOAD
I
L
INPUT OVERLOAD CONSIDERATIONS
Failure of a transducer, faults on input lines, or power supply
sequencing can subject the inputs of an instrumentation ampli-
fier to voltages well beyond their linear range, or even the supply
voltage, so it is essential that the amplifier handle these over-
loads without being damaged.
The AD621 will safely withstand continuous input overloads of
± 3.0 volts (± 6.0 mA). This is true for gains of 10 and 100, with
power on or off.
The inputs of the AD621 are protected by high current capacity
dielectrically isolated 400 Ω thin-film resistors R3 and R4 (Fig-
ure 3) and by diodes which protect the input transistors Q1 and
Q2 from reverse breakdown. If reverse breakdown occurred, there
would be a permanent increase in the amplifier’s input current.
The input overload capability of the AD621 can be easily increased
while only slightly degrading the noise, common-mode rejection
and offset drift of the device by adding external resistors in series
with the amplifier’s inputs as shown in Figure 10.
Table II summarizes the overload voltages and total input
noise for a range of range of r values. Note that a 2 kΩ resis-
tor in series with each input will protect the AD621 from a
± 15 volt continuous overload, while only increasing input noise
to 13 nV√Hz—about the same level as would be expected from
a typical unprotected 3 op amp in amp.
Value of
Resistor R
0
499 Ω
1.00 kΩ
2.00 kΩ
3.01 kΩ
4.99 kΩ
or equivalent.
1/4 watt, 1% metal-film resistor. All others are 1/8 watt, 1% RN55
Table II. Input Overload Protection vs. Value of Resistor R
P
Total Input Noise
in nV√Hz @ 1 kHz
G = 10
14
14
14
15
16
17
G = 100
9
10
11
13
14
16
Maximum Continuous
Overload Voltage, V
In Volts
3
6
9
15
21
33
AD621
OL
P