AMP01
INPUT AND OUTPUT OFFSET VOLTAGES
GAIN
Instrumentation amplifiers have independent offset voltages
associated with the input and output stages. While the initial
offsets may be adjusted to zero, temperature variations will
cause shifts in offsets. Systems with auto-zero can correct for
offset errors, so initial adjustment would be unnecessary. How-
ever, many high-gain applications don’t have auto zero. For
these applications, both offsets can be nulled, which has mini-
mal effect on TCV
IOS
and TCV
OOS
The input offset component is directly multiplied by the ampli-
fier gain, whereas output offset is independent of gain. There-
fore, at low gain, output-offset errors dominate, while at high
gain, input-offset errors dominate. Overall offset voltage, V
OS
,
referred to the output (RTO) is calculated as follows;
V
OS
(RTO) =
(V
IOS
×
G)
+
V
OOS
(1)
where V
IOS
and V
OOS
are the input and output offset voltage
specifications and G is the amplifier gain. Input offset nulling
alone is recommended with amplifiers having fixed gain above
50. Output offset nulling alone is recommended when gain is
fixed at 50 or below.
In applications requiring both initial offsets to be nulled, the
input offset is nulled first by short-circuiting R
G
, then the output
offset is nulled with the short removed.
The overall offset voltage drift TCV
OS
, referred to the output, is
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;
TCV
OS
(RTO) = (TCV
IOS
×
G)
+
TCV
OOS
(2)
where TCV
IOS
is the input offset voltage drift, and TCV
OOS
is
the output offset voltage specification. Frequently, the amplifier
drift is referred back to the input (RTI), which is then equiva-
lent to an input signal change;
TCV
OS
(RTI)
=
TCV
IOS
TCV
OOS
G
The AMP01 uses two external resistors for setting voltage gain
over the range 0.1 to 10,000. The magnitudes of the scale resis-
tor, R
S
, and gain-set resistor, R
G
, are related by the formula:
G = 20
×
R
S
/R
G
, where G is the selected voltage gain (refer to
Figure 29).
V+
R
S
14
+IN
18
1
R
G
2
3
15
13
12
7
SENSE
AMP01
10
9
OUTPUT
8
11 REFERENCE
–IN
VOLTAGE GAIN, G =
(
20 R R
)
S
G
V–
Figure 29. Basic AMP01 Connections for Gains
0.1 to 10,000
(3)
For example, the maximum input-referred drift of an AMP01 EX
set to G = 1000 becomes;
TCV
OS
(RTI ) = 0.3
µ
V/°C
+
100
µV
/°
C
= 0.4
µ
V/°C max
1000
The magnitude of R
S
affects linearity and output referred errors.
Circuit performance is characterized using R
S
= 10 kΩ when
operating on
±
15 volt supplies and driving a
±10
volt output. R
S
may be reduced to 5 kΩ in many applications particularly when
operating on
±
5 volt supplies or if the output voltage swing is
limited to
±
5 volts. Bandwidth is improved with R
S
= 5 kΩ and
this also increases common-mode rejection by approximately
6 dB at low gain. Lowering the value below 5 kΩ can cause
instability in some circuit configurations and usually has no
advantage. High voltage gains between two and ten thousand
would require very low values of R
G
. For R
S
= 10 kΩ and
A
V
= 2000 we get R
G
= 100
Ω;
this value is the practical lower
limit for R
G
. Below 100
Ω,
mismatch of wirebond and resistor
temperature coefficients will introduce significant gain tempco
errors. Therefore, for gains above 2,000, R
G
should be kept
constant at 100
Ω
and R
S
increased. The maximum gain of
10,000 is obtained with R
S
set to 50 kΩ.
Metal-film or wirewound resistors are recommended for best
results. The absolute values and TCs are not too important,
only the ratiometric parameters.
AC amplifiers require good gain stability with temperature and
time, but dc performance is unimportant. Therefore, low cost
metal-film types with TCs of 50 ppm/°C are usually adequate
for R
S
and R
G
. Realizing the full potential of the AMP01’s offset
voltage and gain stability requires precision metal-film or wire-
wound resistors. Achieving a 15 ppm/°C gain tempco at all gains
requires R
S
and R
G
temperature coefficient matching to
5 ppm/°C or better.
INPUT BIAS AND OFFSET CURRENTS
Input transistor bias currents are additional error sources that
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 a nontrimmable
error. The magnitude 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.
REV. D
–11–
AD [ ANALOG DEVICES ]
