ISL28533 Datasheet, PDF (27/30 Pages) Intersil Corporation – 5V, Rail-Rail I/O, Zero-Drift, Programmable Gain Instrumentation Amplifiers

ISL28533, ISL28534, ISL28535, ISL28633, ISL28634, ISL28635
GAIN SWITCHING DELAY TIME
The G0 and G1 pins change the gain setting of the PGIA. For
applications that must switch gains at high frequency, consider
that there is a gain switching propagation delay of ~1µs before
output response. The total response time for a gain change must
also include the amplifier output settling time. See “Electrical
Specifications” starting on page 6 for output settling time.
DUAL SUPPLY OPERATION
ISL2853X and ISL2863X typical applications utilize single supply
operation. The single supply range is from 2.5V to 5V, but the
amplifiers can also operate with split supplies from ±1.25V to
±2.5V. The G0 and G1 logic thresholds are referenced to the
most negative supply rail (V-), therefore a logic level shifter is
needed in split supply applications when the G0 and G1 pins are
not strapped to the amplifier supply pins (i.e., when driven by a
single supply logic device).
POWER SUPPLY AND REF PIN SEQUENCING
As the REF pin in some applications is tied to a high accuracy
voltage reference VREF (such as the ISL21090), proper care
must be taken that the voltage at REF does not come up prior to
supply voltages V+ and V-. The REF pin ESD protection diodes will
be forward biased when the voltage at REF exceeds V+ or V- by
more than 0.3V. For applications where REF must be present
before V+ or V-, it is recommended to use the ISL2863x family of
PGIA. As the REF pin is an very high impedance input, having a
series resistance to limit the ESD diode current will not severely
impact CMRR performance. Typically a 1kΩ resistor will
adequately limit this current.
COMMON MODE INPUT RANGE
The 3-Op Amp Instrumentation Amplifier architecture amplifies
differential input voltage. The common mode voltage is removed
by the difference amplifier at the second stage. Consideration of
input common mode and differential voltage must be taken to
not saturate the output of the A1 and A2 amplifiers. This is a
common mistake when input differential voltages plus the input
V_CM combined is large enough to saturate the output. The PGIA
features rail to rail output amplifiers to maximize output dynamic
range thus signals VA+, VA- and VOUT+/VOUT- can drive near the
supply rails. Figures 17 to 20 give the typical input common
mode voltage range vs output voltage for different REF voltages.
Application Circuits
Typical application circuits for bridge sensor health monitor and
active shield guard driver are shown in Figures 77 and 78.
Sensor Health Monitor
A bridge type sensor uses four matched resistive elements to
create a balanced differential circuit. The bridge can be a
combination of discrete resistors and resistive sensors for a
quarter, half and full bridge applications. The bridge is excited by a
low noise, high accuracy voltage reference or current source on
two legs. The other two legs are the differential signal whose
output voltage change is analogous to changes in the sensed
environment. In a bridge circuit, the common mode voltage of the
differential signal is at the mid point potential voltage of the bridge
excitation source. For example in a single supply system using a
+5V reference for excitation, the common mode voltage is +2.5V.
The concept of sensor health monitoring is to keep track of the
bridge impedance within the data acquisition system. Changes in
the environment, degradation over time or a faulty bridge
resistive element will imbalance the bridge, causing
measurement errors. Since the bridge differential output
common mode voltage is one-half the excitation voltage, by
measuring this common mode the sensor impedance health can
be monitored, for example through an ADC channel (see
Figure 77). While common mode voltage can be measured
directly off the bridge, this is not recommended because the
bridge impedance is highly sensitive to any additional loading.
Sensing off the legs directly can give an erroneous reading of the
analog signal being measured. Since the VA+ and VA- pins buffer
the input common mode voltage, this provides a low impedance
point to drive the ADC without using additional amplifiers. By
continuously monitoring the common mode voltage this gives an
indication of sensor health.
Active Shield Guard Drive
Sensors that operate at far distances from the signal
conditioning circuits are subject to noise environments that
reduce the signal to noise ratio into an amplifier. Differential
signaling and shielded cables are a few techniques that are used
to reduce noise from sensitive signal lines. Reducing noise that
the instrumentation amplifier cannot reject (high frequency noise
or common mode voltage levels beyond supply rail) improves
measuring accuracy. Shielded cables offer excellent rejection of
noise coupling into signal lines. However, cable impedance
mismatch to signal wires form a common mode error into the
amplifier. Driving the cable shield to a low impedance potential
reduces the impedance mismatch. The cable shield is usually
tied to chassis ground as it makes an excellent low impedance
point and is easily accessible. However, this may not always be
the best potential voltage to tie the shield to, in particular for
single supply amplifiers.
In some data acquisition systems the sensor signal amplifiers
are powered with dual supplies (±5V or ±12V). By tying the shield
to analog ground 0V, this places the common mode voltage of
the shield right at the middle of the supply bias - where the
amplifiers operate with the best CMR performance. With single
supply amplifiers becoming more popular choice as a sensor
amplifier, shield at 0V is now at the lower power supply rail of the
amplifier - typically a common mode voltage where the same
CMR performance degrades. Tying the shield at common mode
voltage of mid supply rail is most applicable for high impedance
sensor applications.
An alternative solution for an improved shield guard drive is to
use the VA+ and VA- pins for sensing common mode and driving
the shield to this voltage (see Figure 78). Using the VA+ and VA-
pins generate a low impedance reference of the input common
mode voltage. Driving the shield to the input common mode
voltage reduces cable impedance mismatch and improves CMR
performance in single supply sensor applications. For further
buffering of the shield driver, the additional unused op amp on
the ISL2853x products can be used, reducing the need of adding
an external amplifier.
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FN8364.0
September 24, 2013