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使用仪器仪表放大器时需要防止的5个错误
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The in amp is a vital signal-processing
component, but there are some basic application do's and don'ts that
can degrade their performance
Instrumentation amplifiers (in-amps) show up in a broad spectrum of
applications: measuring heart signals, factory monitoring equipment,
aircraft controls, and even animal tagging. Engineers have found them
to be a simple and effective way to amplify small signals and remove
power-line noise. Unlike ADCs, with their modes and registers, a
typical in-amp has only two adjustments: gain and reference voltage.
Unlike op amps, where poor feedback design means oscillation, in-amps
are quite stable.
The instrumentation amplifier's ease of use can lead to a sense of
complacency. While it is easy to get an in-amp up and running on the
bench, poor attention to detail can lead to mediocre performance in the
field. Since the in-amp is typically connected directly to a sensor,
designers must think about the full range of signals this sensor could
present. This article will cover five cases that often trip up
designers.
RFI rectification
Instrumentation amplifiers are often connected to an external sensor
through leads of several feet or more. These leads act as a sensitive
antenna: picking up both 50/60 Hz noise as well as much higher
frequencies.
Most modern monolithic
instrumentation amplifiers reject almost all 50/60 Hz common-mode
noise. In fact, common-mode rejection (CMR) is a key specification and
is prominently displayed in most in-amp datasheets. It is typically
specified from dc
to 60 Hz. Depending on the architecture, an in-amp may be good at
rejecting mid-frequency interference. An example of good mid-frequency
rejection is 80 dB CMR at 10 kHz, which is the performance of Analog Devices' AD8221 in-amp.
Modern monolithic amplifiers are not very good at rejecting higher common-mode frequencies, Figure 1.
Instrumentation amplifiers are
designed to be low-current, precise devices. The tradeoff for the low
current and precision is speed. Most instrumentation amplifiers are
simply not fast enough to faithfully track fast common-mode signals.
Not only do instrumentation
amplifiers have a hard time reducing the amplitude of the
high-frequency signals, they also distort them as they pass through the
amplifier. This distortion, known as radio frequency interference (RFI)
rectification, can create lower-frequency products in the band of
interest. This is why high-frequency ac content can result in dc offsets.
The solution is to prevent the high
frequencies from reaching the in-amp in the first place. This can be
achieved by placing a low-pass filter before the instrumentation
amplifier. Figure 2 shows such a filter.
high-frequency signals before they are rectified by the amplifier's
front end.
Resistors should be chosen to meet noise versus voltage protection tradeoffs (see Voltage Protection section below). The differential capacitor (CD)
should be chosen to place the differential frequency cutoff slightly
higher than the signal of interest. Finally, the common-mode capacitors
(CC) should be large enough so that the low pass filter formed by R and CC adequately rejects the RFI interference.
The CC capacitors should
match each other as closely as possible. Any mismatch will result in
different low-pass filter characteristics for the two inputs. Even
slight mismatches are enough to reduce mid-frequency CMR. Since smaller
capacitor values create less absolute mismatch, the choice of CC capacitors is a tradeoff between RFI filtering protection and CMR at mid-frequencies.
A typical guideline is to make the CC capacitors at least 10 times smaller than the CD capacitor. For the CC
capacitors, high-accuracy COG type capacitors are recommended over X7R
types, because the values will track each other more closely. The
accuracy of the CD capacitor is much less critical.
Note that in some cases of very strong RFI signals, the interference may not come in through the input
leads but, instead, through other pins or through the package itself.
In these cases, the problem may need to be addressed at the source of
the interference or through shielding.
Common mode range
Just like op amps, in-amp datasheets have entries in the specification table titled "Input Voltage Range" and "Output Voltage Range", or something similar. This can be misleading. Even if the circuit is designed to keep the input and output signals inside their respective ranges, the amplifier may still not function as expected.
Most instrumentation amplifiers
have two stages: a preamp stage, which amplifies the differential
signal, and a difference-amplifier stage, which then removes the common
mode. In between the two stages are nodes that must carry the
combination of both the amplified differential signal and the common
mode. It's possible these nodes can reach the supply rail value, even
when the input and output are within their specified ranges.
Figure 3 shows how this behavior plays out with actual supplies and input voltages.
depends on the architecture; shown is the typical behavior of the two
most-common architectures: the three op-amp and two op-amp in amp.
The two diagrams show the
typical behavior of the most common in-amp architectures: the two
op-amp configuration and the three op-amp configuration. The figures
show the behavior of these architectures assuming ideal rail-to-rail op
amps were used, as well as some actual monolithic parts.
How can a board designer know the
true common-mode range? Older in-amp datasheets are remarkably silent
on this issue, and it caused a lot of design headaches for in-amp
users. Modern in-amp datasheets typically include graphs similar to
those shown in Figure 3, or they provide equations. To test the
common-mode range on the bench, increase the common-mode voltage until
the circuit gain seems to decrease. At this point, the voltage on one
of the internal nodes has reached its limit.
As illustrated in Figure 3,
different in-amps have different behavior. So, if one amplifier does
not work in a specific application, try another. Another option is to
reduce the gain of the instrumentation amplifier and compensate by
applying more gain later in the signal chain. This strategy can
adversely affect the system noise, CMR, and offset performance,
however, so read the datasheet carefully.
It should be noted that IC
manufacturers are developing architectures that do not have this
internal node limitation. The AD8553 in-amp from Analog Devices is an
example of such a product. One drawback of these new architectures is
that they tend to have poorer noise performance at low gains than the
traditional architectures.
Voltage Protection
In amps are often connected to sensors outside the circuit board. In
actual use, these connections may be made to the wrong place,
subjecting the in amp to voltages larger than intended. If the in amp
is left unprotected, these large voltages will damage the amplifier.
It is often not the voltages, but
the large currents created by these voltages, that cause the failure. A
primary failure mechanism is metal migration: when the small metal
interconnects in the chip erode away under high-current conditions.
Metal migration is a cumulative process and is accelerated with heat,
so amplifiers can typically tolerate shorts bursts of high voltages
better than prolonged exposure.
The classic way to protect a
circuit from overvoltages is to add resistors in series with the
inputs. If the datasheet provides no other information, it is generally
safe to assume that ESD protection diodes can handle at least 5 mA of
current. The resistors should be chosen so that the maximum voltage
drop between the input voltage and the in-amp supply causes less than 5
mA to flow into these diodes.
Unfortunately, resistors add noise.
A 1 kΩ resistor has 4 nV per root Hz of thermal noise; a 100 kΩ
resistor has 40 nV per root Hz of noise. It is important to remember
that noise adds by the square root of the sum of squares: √(en12 + en22 + en32),
The total impact on the system noise is insignificant if the resistor
noise is three to four times lower than the intrinsic in-amp noise.
In some cases it may be possible to
use low-value resistors and then discrete diodes between the inputs and
supplies to shunt current away from the part. The success of this
strategy is highly dependent on the amplifier's input structure. Before
trusting such a circuit in full production, it is important to
determine on a prototype where the current flows during overvoltage
conditions. Low-leakage, low-capacitance diodes are recommended to keep
overall input error at a minimum.
Driving the Reference Pin
An in amp's output is created with respect to the voltage at the
reference pin. This can be quite handy in single-supply applications,
where the signal should be centered at mid-supply. Simply drive the
reference pin to the required bias voltage.
For the vast majority of in amp,
the reference pin must be connected to a low impedance: either ground
or an amplifier output. A resistor divider simply won't do. Driving
from a high-impedance source will result in poor CMR, Figure 4.
drive the reference pin with a low-impedance source or ground. Never
tie the reference pin directly to a voltage divider.
A good guideline is to keep the dc impedance below 1 Ω.
Floating Voltages
Because an in amp has both a + and - terminal, it might be tempting to
think of it as something like a mini-version of a handheld voltmeter.
Unlike an isolated, handheld voltmeter, however, an in amp cannot
measure floating voltages. This includes anything that is not
referenced to ground: isolated thermocouples, secondary sides of
transformers, and batteries. The bias currents of the in amp will pull
any floating source out of the amplifier's common-mode range. This is
what the data sheet means when it states that the amplifier needs an
input bias-current return path.
A corollary rule is that an in amp
with its inputs shorted does not necessarily produce zero volts at the
output. The shorted inputs must also be connected to a voltage in the
common-mode range of the amplifier. Figure 5 shows ideas for typical floating sources.
Because in amps are easy to use,
they can usually be treated as simple black boxes. This article covered
the five most common cases where a little deeper understanding can
result in a more robust and reliable design. The Reference provides more detailed information about how instrumentation amplifiers work.
