Datasheet
A
3
50 kW
50 kW
10 kW
10 kW
V+
V-
V
OUT
V+
V-
REF
Q
2
V+
V-
Overvoltage
Protection
A
2
R
B
C
2
V
B
V-
R
2
8 kW
A
1
R
B
C
1
R
1
8 kW
R
(External)
G
V+
V-
V+
V-
V+
V-
Q
1
Overvoltage
Protection
-IN
V /2
D
V /2
D
V
CM
+IN
A Out = V + V + 0.125 V V /2 G
A Out = V + V + 0.125 V + V /2
Output Swing Range A , A , (V+) 0.1 V to (V ) + 0.1 V
´
1 CM BE D
2 CM BE D
1 2
-
´
- -
G
V = G (V V ) + V
Linear Input Range A = (V+) 0.9 V to (V ) + 0.1 V
O IN+ IN REF
3
-
´ -
- -
INA827
SBOS631A –JUNE 2012–REVISED JULY 2013
www.ti.com
INSIDE THE INA827
Refer to Figure 54 for a simplified representation of the INA827. A more detailed diagram (shown in Figure 56)
provides additional insight into the INA827 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 Q
1
and Q
2
and is applied across R
G
, causing a signal current to flow
through R
G
, R
1
, and R
2
. The output difference amplifier (A
3
) removes the common-mode component of the input
signal and refers the output signal to the REF terminal.
The equations shown in Figure 56 describe the output voltages of A
1
and A
2
. The V
BE
and voltage drop across
R
1
and R
2
produce output voltages on A
1
and A
2
that are approximately 0.8 V higher than the input voltages.
Figure 56. INA827 Simplified Circuit Diagram
INPUT PROTECTION
The INA827 inputs 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 Figure 14. This polarity reversal can easily be avoided by adding a 10-kΩ resistance 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. Figure 14
illustrates this input current limit behavior. The inputs are protected even if the power supplies are disconnected
or turned off.
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