ISL28617 Datasheet, PDF (13/18 Pages) Intersil Corporation – 40V Precision Instrumentation Amplifier with Differential ADC Driver

ISL28617
The gain of the instrumentation amplifier is set by the resistor
ratio R_FB/R_IN (Equation 7), and the maximum output swing is set
by the absolute value of the feedback resistor R_FB (Equation 8).
V_CO and V_EO supply power to the rail-to-rail output stage and
define the maximum output voltage swing at the ±V_OUT
différentiel output pins. Power supply pins V_CC and V_EE power the
feedback amplifiers which, require an additional ±3V beyond the
V_CO and V_EO voltages to maintain linear operation of the
feedback G_M stage.
Setting the Feedback Gain Resistor R_FB
(Figures 29, 30)
Resistor R_FB defines the maximum differential voltage at output
terminals +V_OUT to -V_OUT. External resistor R_FB and the differential
100µA current sources define the maximum dynamic range of
the feedback stage, which defines the maximum differential
output swing of the output stage. Overload circuitry allows
>100µA to flow through R_FB to maintain feedback, but linearity is
degraded. Therefore, it is a good practice to keep the maximum
linear dynamic range to within ±80% of the maximum I*R across
the resistor.
V_OUTDIFF= ±80µA * R_FB
(EQ. 8)
In cases where large pulse overshoot is expected, the maximum
current in Equation 8 could be reduced to 50% for additional
margin (See “AC Performance Considerations” on page 15.) The
penalty for increasing the feedback resistor value is higher DC
offset voltage and noise.
Output voltages that exceed the maximum dynamic range of the
feedback amplifier can degrade phase margin and cause
instability. The plot in Figure 30 shows the maximum differential
output voltage swing vs. resistor value for R_FB and R_IN using the
80% and 50% current source levels.
35
30
25
20
15
10
5
0
0
V_OUT (V) @ 80%
V_OUT (V) @ 50%
50 100 150 200 250 300 350 400
R_FB, R_IN VALUE (kΩ)
FIGURE 30. R_FB, R_IN vs. DYNAMIC RANGE
Setting the Input Gain Resistor R_IN
(Figures 29, 30)
The input gain resistor R_IN is scaled to the feedback resistor
according to the gain Equation 9:
R_IN = R_FB/Gain
(EQ. 9)
The input G_M stage uses the same differential current source
arrangement as the feedback stage. Therefore, the amount of
overdrive margin (50%, 80%) included in the calculation for R_FB
is also included in the calculation for R_IN.
Input Stage Overdrive Considerations
(Figure 31)
There are a few cases where the input stage can be over driven,
which must be considered in the application. An input signal that
exceeds the maximum dynamic range of the gain resistor R_IN,
calculated previously, can cause the ESD diodes to conduct.
When this occurs, a low impedance path from the inputs to the
input gain resistor R_IN will result in signal distortion.
High speed input signals that remain within the maximum
dynamic range of the input stage can cause distortion if the input
slew rate exceeds the input stage slew rate (~4V/µs). When the
input slews at a faster rate than the G_M stage can follow, the
voltage difference appears across the input ESD diodes from
each input and resistor R_IN. When the voltage difference is large
enough to cause the diodes to conduct, the input terminals are
shunted to R_IN through the 500Ω input protection resistors,
causing distortion during the rise and fall times of the transient
pulse. The distortion will last until the resistor voltage catches up
to the input voltage.
500Ω
IN-
V_CC
500Ω
IN+
+
-A1
Q1 Q2
R_IN
A2+-
ESD
PROTECTION
100µA 100µA
ESD
PROTECTION
V_EE
FIGURE 31. INPUT STAGE ESD PROTECTION DIODES
Setting the Power Supply Voltages
The ISL28617 power supplies are partitioned so that the input
stage and feedback stages are powered from a separate pair of
supply pins (V_CC, V_EE) than the differential output stage (V_CO,
V_EO). This partitioning provides the user with the ability to adapt
the ISL28617 to a wide variety of input signal power sources that
would not be possible if the supplies were strapped together
internally (V_CC = V_CO & V_EE = V_EO). However, powering the input
and output supplies from unequal supplies has restrictions that
are described in the next section.
Powering the Input and Feedback Stages
(V_CC, V_EE)
The input pins IN+, IN- cannot swing rail-to-rail, but have a
maximum input voltage range given by Equation 10:
V_EE+ 3V < (V_CMIRIN + V_IN) < V_CC - 3V;
where V_IN = maximum differential voltage IN+ to IN-
(EQ. 10)
This requires the sum of the common mode input voltage and
the differential input voltage to remain within 3V of either the V_CC
or V_EE rail, otherwise distortion will result.
13
FN6562.0
May 25, 2012