INA826_1112 Datasheet, PDF (20/40 Pages) Texas Instruments – Precision, 200-μA Supply Current, 2.7-V to 36-V Supply Instrumentation Amplifier with Rail-to-Rail Output

INA826
SBOS562B – AUGUST 2011 – REVISED DECEMBER 2011
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INSIDE THE INA826
See Figure 58 for a simplified representation of the INA826. A more detailed diagram (shown in Figure 60)
provides additional insight into the INA826 operation.
Each input is protected by two field-effect transistors (FETs) that provide a low series resistance under normal
signal conditions, and preserve excellent noise performance. When excessive voltage is applied, these
transistors limit input current to approximately 8 mA.
The differential input voltage is buffered by Q₁ and Q₂ and is impressed across R_G, causing a signal current to
flow through R_G, R₁, and R₂. The output difference amp, A₃, removes the common-mode component of the input
signal and refers the output signal to the REF terminal.
The equations shown in Figure 60 describe the output voltages of A₁ and A₂. The V_BE and voltage drop across
R₁ and R₂ produce output voltages on A₁ and A₂ that are approximately 0.8 V higher than the input voltages.
V+
V+
R_G
(External)
A₁ Out = V_CM + V_BE + 0.125 V - V_D/2 ´ G
A₂ Out = V_CM + V_BE + 0.125 V + V_D/2 ´ G
Output Swing Range A₁, A₂, (V+) - 0.1 V to (V-) + 0.1 V
V_O = G ´ (V_IN+ - V_IN-) + V_REF
Linear Input Range A₃ = (V+) - 0.9 V to (V-) + 0.1 V
R₁
24.7 kW
V-
R₂
V-
24.7 kW
50 kW
50 kW
50 kW
V+
A₃
V+
50 kW
V-
REF
V_OUT
V+
-IN
V_D/2
Overvoltage
V-
Protection
Q₁
C₁
A₁
Q₂
A₂
C₂
V-
V+
V-
Overvoltage
Protection
V_CM
V_D/2
+IN
R_B
V_B
R_B
V-
Figure 60. INA826 Simplified Circuit Diagram
INPUT PROTECTION
The inputs of the INA826 are individually protected for voltages up to ±40 V. For example, a condition of –40 V
on one input and +40 V on the other input does not cause damage. However, if the input voltage exceeds (V–) –
2 V and the signal source current drive capability exceeds 3.5 mA, the output voltage switches to the opposite
polarity; see typical characteristic curve Input Overvoltage vs Input Current (Figure 17). This polarity reversal can
easily be avoided by adding resistance of 10 kΩ in series with both inputs.
Internal circuitry on each input provides low series impedance under normal signal conditions. If the input is
overloaded, the protection circuitry limits the input current to a safe value of approximately 8 mA. The typical
characteristic curves Input Current vs Input Overvoltage (Figure 17 and Figure 18) illustrate this input current limit
behavior. The inputs are protected even if the power supplies are disconnected or turned off.
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