amp02az-883c Analog Devices, Inc., amp02az-883c Datasheet - Page 8

• Manufacturer Part Number: amp02az-883c
• Description: High Accuracy 8-pin Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

AMP02
APPLICATIONS INFORMATION
INPUT AND OUTPUT OFFSET VOLTAGES
Instrumentation amplifiers have independent offset voltages
associated with the input and output stages. The input offset
component is directly multiplied by the amplifier gain, whereas
output offset is independent of gain. Therefore, at low gain,
output-offset-errors dominate, while at high gain, input-offset-
errors dominate. Overall offset voltage, V
put (RTO) is calculated as follows:
where V
specifications and G is the amplifier gain.
The overall offset voltage drift TCV
a combination of input and output drift specifications. Input
offset voltage drift is multiplied by the amplifier gain, G, and
summed with the output offset drift:
where TCV
the output offset voltage drift. Frequently, the amplifier drift is
referred back to the input (RTI) which is then equivalent to an
input signal change:
For example, the maximum input-referred drift of an
AMP02EP set to G = 1000 becomes:
TCV
INPUT BIAS AND OFFSET CURRENTS
Input transistor bias currents are additional error sources which
can degrade the input signal. Bias currents flowing through the
signal source resistance appear as an additional offset voltage.
Equal source resistance on both inputs of an IA will minimize
offset changes due to bias current variations with signal voltage
and temperature. However, the difference between the two bias
currents, the input offset current, produces an error. The mag-
nitude of the error is the offset current times the source resistance.
A current path must always be provided between the differential
inputs and analog ground to ensure correct amplifier operation.
Floating inputs, such as thermocouples, should be grounded
close to the signal source for best common-mode rejection.
GAIN
The AMP02 only requires a single external resistor to set the
voltage gain. The voltage gain, G, is:
and
OS
(RTI) = 2 V/ C +
IOS
TCV
and V
IOS
TCV
is the input offset voltage drift, and TCV
V
OS
OS
OOS
(RTO) = (TCV
OS
(RTO) = (V
are the input and output offset voltage
(RTI) =TCV
G
R
100 V / C
G
=
50 k
1000
R
50 k
IOS
G
G –1
IOS
IOS
OS
1
, referred to the output, is
G) + V
+
= 2.1 V/ C
G) + TCV
TCV
OS
, referred to the out-
G
OOS
OOS
OOS
OOS
is
–8–
The voltage gain can range from 1 to 10,000. A gain set resistor
is not required for unity-gain applications. Metal-film or wire-
wound resistors are recommended for best results.
The total gain accuracy of the AMP02 is determined by the tol-
erance of the external gain set resistor, R
gain equation accuracy of the AMP02. Total gain drift com-
bines the mismatch of the external gain set resistor drift with
that of the internal resistors (20 ppm/ C typ). Maximum gain
drift of the AMP02 independent of the external gain set resistor
is 50 ppm/ C.
All instrumentation amplifiers require attention to layout so
thermocouple effects are minimized. Thermocouples formed be-
tween copper and dissimilar metals can easily destroy the
TCV
0.5 V/ C. Resistors themselves can generate thermoelectric
EMFs when mounted parallel to a thermal gradient.
The AMP02 uses the triple op amp instrumentation amplifier
configuration with the input stage consisting of two transimped-
ance amplifiers followed by a unity-gain differential amplifier.
The input stage and output buffer are laser-trimmed to increase
gain accuracy. The AMP02 maintains wide bandwidth at all
gains as shown in Figure 26. For voltage gains greater than 10,
the bandwidth is over 200 kHz. At unity-gain, the bandwidth of
the AMP02 exceeds 1 MHz.
COMMON-MODE REJECTION
Ideally, an instrumentation amplifier responds only to the differ-
ence between the two input signals and rejects common-mode
voltages and noise. In practice, there is a small change in output
voltage when both inputs experience the same common-mode
voltage change; the ratio of these voltages is called the common-
mode gain. Common-mode rejection (CMR) is the logarithm
of the ratio of differential-mode gain to common-mode gain, ex-
pressed in dB. Laser trimming is used to achieve the high CMR
of the AMP02.
Figure 26. The AMP02 Keeps Its Bandwidth at
High Gains
OS
performance of the AMP02 which is typically
G
, combined with the
REV. D