ISL70517SEH Datasheet, PDF (18/27 Pages) Intersil Corporation – Precision test and measurement

ISL70517SEH
Functional Description
Figure 48 on page 17 shows the functional block diagram for the
ISL70517SEH.
Input G_M Amplifier
The input stage consists of high performance, wideband
amplifiers (A1, A2), G_M drive transistors (Q1, Q2), and input gain
resistor (R_IN). Current drive for Q1 and Q2 emitters are provided
by a matched pair of 100µA current sinks. A unity gain buffer
from each input (IN+, IN-) to the terminals of the input resistor,
R_IN, is formed by the connection of the Kelvin resistor SENSE pins
and drive pins to the terminals of the input resistor, as shown in
Figure 48. In this configuration, the voltage across the input
resistor R_IN is equal to the input differential voltage across IN+
and IN-.
The input G_M stage operates by creating a current difference in
the collector currents Q1 and Q2 in response to the voltage
difference between the IN+ and IN- pins. When the input voltage
applied to the IN+ and IN- pins is zero, the voltage across the
terminals of the gain resistor R_IN, is also zero. Since there is no
current flow through the gain resistor, the transistors Q1 and Q2
collector currents (I₁, I₂) are equal.
A change in the input differential voltage causes an equivalent
voltage drop across the input gain resistor R_IN and the resulting
current flow through R_IN causes an imbalance in Q1, Q2 collector
currents I₁, I₂, given by Equations 1 and 2:
I₁= 100A + V_IN+ – V_IN-  R_IN
(EQ. 1)
I₂= 100A – V_IN+ – V_IN-  R_IN
(EQ. 2)
Feedback G_M Amplifier
The feedback amplifiers A3, A4 form a differential
transconductance amplifier identical to the input stage. The
input terminals (V_FB+, V_FB-) connect to the ISL70517SEH
differential output terminals (+V_OUT, -V_OUT) so that the output
voltage also appears across the feedback gain resistor R_FB.
Operation is the same as the input G_M stage and the differential
currents I₃, I₄ are given by Equations 3 and 4:
I₃= 100A + +V_OUT – -V_OUT  R_FB
(EQ. 3)
I₄= 100A + +V_OUT – -V_OUT  R_FB
(EQ. 4)
Error Amplifier A5, Output Amplifier A6
Amplifiers A5 and A6 act together to form a high-gain,
differential I/O transimpedance amplifier (refer to Figure 48).
Differential current amplifier A5 sums the differential currents (I₁
+ I₃, I₂ + I₄) from the input and feedback G_M amplifiers. From
that summation, a differential error voltage is sent to A6, which
generates the rail-to-rail differential output drive to the +V_OUT and
-V_OUT pins.
The external connection of the output pins to the feedback
amplifier closes a servo loop where a change in the differential
input voltage is converted into differential current imbalances at
I₁, I₂ (Equations 1 and 2) at the summing node inputs to A5.
Current I₁ sums with current I₃ from the feedback stage and I₂
sums with I₄. A5 senses the difference between current pairs I₁,
I₃ and I₂, I₄. A differential voltage is generated, amplified, and
fed back to the feedback amplifier, which creates correction
currents at I₃, I₄ to match the currents at I₁, I₂ (Equations 3 and
4).
Therefore, at equilibrium:
I₁= I₃and I₂= I₄
(EQ. 5)
Combining Equations 1 and 3, (and their complements I₂ and I₄)
and solving for V_OUT as a function of V_IN, R_IN, and R_FB, yields
Equation 6:
V_OUT= V_IN^R_FB  R_IN;
where V_OUT = +V_OUT – -V_OUTand V_IN = IN+ – IN-
(EQ. 6)
Equation 6 can be rearranged to form the gain Equation 7:
Gain= V_OUT  V_IN = R_FB  R_IN
(EQ. 7)
This is a general form of the gain equation for the ISL70517SEH.
Designing with the ISL70517SEH
To complete a working design, the following procedure is
recommended and explained in this section:
1. Define the output voltage swing
2. Set the feedback resistor value, R_FB (Equation 8)
3. Set the input gain resistor value, R_IN
4. Set the V_CO and V_EO power supply voltages
5. Set the V_CC and V_EE supply voltages
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).
The 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
differential 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)
Resistor R_FB defines the maximum differential voltage at output
terminals +V_OUT to -V_OUT (refer to Figures 48 and 49). 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)
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FN8699.4
December 15, 2016