AMP01 查看數據表（PDF） - Analog Devices
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AMP01 Datasheet PDF : 30 Pages
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Data Sheet
AMP01
Metal film or wire wound 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 RS and RG. Realizing the full potential of the offset voltage
and gain stability of the AMP01 requires precision metal film or
wire wound resistors. Achieving a 15 ppm/°C gain TC at all
gains requires RS and RG temperature coefficient matching to
5 ppm/°C or better.
1M
VS = ±15V
100k
10k
RS
RG
1k
100
1
10
100
1k
10k
VOLTAGE GAIN
Figure 33. RG and RS Selection
Gain accuracy is determined by the ratio accuracy of RS and RG
combined with the gain equation error of the AMP01 (0.6%
maximum for A and E grades).
All instrumentation amplifiers require attention to layout so that
thermocouple effects are minimized. Thermocouples formed
between copper and dissimilar metals can destroy the TCVOS
performance of the AMP01, which is typically 0.15 μV/°C.
Resistors themselves can generate thermoelectric EMFs when
mounted parallel to a thermal gradient. Vishay resistors are
recommended because a maximum value for thermoelectric
generation is specified. However, where thermal gradients are
low and gain TCs of 20 ppm to 50 ppm are sufficient, general-
purpose metal film resistors can be used for RG and RS.
COMMON-MODE REJECTION
Ideally, an instrumentation amplifier responds only to the
difference 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, expressed in dB. CMR specifications are normally
measured with a full-range input voltage change and a specified
source resistance unbalance.
The current feedback design used in the AMP01 inherently
yields high common-mode rejection. Unlike resistive feedback
designs, typified by the 3-op-amp IA, the CMR is not degraded
by small resistances in series with the reference input. A slight
but trimmable output offset voltage change results from
resistance in series with the reference input.
The common-mode input voltage range (CMVR) for linear
operation can be calculated from the formula,
CMVR IVR |VOUT |
(4)
2G
where:
IVR is the data sheet specification for the input voltage range.
VOUT is the maximum output signal.
G is the chosen voltage gain.
For example, at 25°C, IVR is specified as ±10.5 V minimum
with ±15 V supplies. Using a ±10 V maximum swing output and
substituting the figures in Equation 4 simplifies the formula to
CMVR
10.5
5
G
(5)
For all gains greater than or equal to 10, CMVR is ±10 V
minimum; at gains below 10, CMVR is reduced.
ACTIVE GUARD DRIVE
Rejection of common-mode noise and line pickup can be improved
by using shielded cable between the signal source and the IA.
Shielding reduces pickup, but increases input capacitance, which in
turn degrades the settling-time for signal changes. Furthermore,
any imbalance in the source resistance between the inverting
and noninverting inputs, when capacitively loaded, converts the
common-mode voltage into a differential voltage. This effect
reduces the benefits of shielding. AC common-mode rejection is
improved by bootstrapping the input cable capacitance to the input
signal, a technique called guard driving. This technique effectively
reduces the input capacitance. A single guard-driving signal is
adequate at gains above 100 and must be the average value of
the two inputs. The value of the external gain resistor, RG, is split
between two resistors, RG1 and RG2; the center tap provides the
required signal to drive the buffer amplifier (see Figure 34).
GROUNDING
The majority of instruments and data acquisition systems have
separate grounds for analog and digital signals. Analog ground
can also be divided into two or more grounds that are tied
together at one point, usually the analog power-supply ground.
In addition, the digital and analog grounds can be joined,
normally at the analog ground pin on the analog-to-digital
converter (ADC). Following this basic grounding practice is
essential for good circuit performance (see Figure 35).
Mixing grounds causes interactions between digital circuits and
the analog signals. Because the ground returns have finite
resistance and inductance, hundreds of millivolts can be
developed between the system ground and the data acquisition
components. Using separate ground returns minimizes the
current flow in the sensitive analog return path to the system
Rev. E | Page 19 of 29
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