10

®

INA156

FIGURE 6. Input Offset Voltage Changes with Common-

Mode Voltage.

FIGURE 7. Single-Supply, High-Side Current Monitor.

FIGURE 8. Input Current Protection for Voltages Exceed-

ing the Supply Voltage.

FIGURE 9. Driving Capacitive-Input A/D Converter.

INPUT COMMON-MODE RANGE

The input common-mode range of the INA156 for various

operating conditions is shown in the Typical Performance

Curves. The common-mode input range is limited by the

output voltage swing of A1, an internal circuit node. For the

G = 10 configuration, output voltage of A1 can be expressed as:

VOUTA1 = – 1/9VREF + (1 + 1/9) VIN–

(2)

The input common-mode voltage range can be calculated

using this equation, given that the output of A1 can swing to

within 20mV of either rail. When the input common-mode

range is exceeded (A1’s output is saturated), A2 can still be in

linear operation and respond to changes in the non-inverting

input voltage. However, the output voltage will be invalid.

The common-mode range for the G = 50 configuration is

included in the typical performance curve, “Input Common-

Mode Range vs Output Voltage.”

INPUT RANGE FOR BEST ACCURACY

The internal amplifiers have rail-to-rail input stages, achieved

by using complementary n-channel and p-channel input

pairs. The common-mode input voltage determines whether

the p-channel or the n-channel input stage is operating. The

transition between the input stages is gradual and occurs

between (V+) – 1.8V to (V+) – 1V. Due to these character-

istics, operating the INA156 with input voltages within the

transition region of (V+) – 1.8V to (V+) – 0.8V results in a

shift in input offset voltage, and reduced common-mode and

power supply rejection performance. Typical patterns of the

offset voltage change throughout the input common-mode

range are illustrated in Figure 6.

The INA156 can be

operated below or above the transition region with excellent

results. Figure 7 demonstrates the use of the INA156 in a

single-supply, high-side current monitor. In this application,

the INA156 is operated above the transition region.

RAIL-TO-RAIL OUTPUT

A class AB output stage with common-source transistors is

used to achieve rail-to-rail output. For resistive loads

greater than 10k

Ω, the output voltage can swing to within

a few millivolts of the supply rail while maintaining low

gain error. For heavier loads and over temperature, see the

typical performance curve “Output Voltage Swing vs Out-

put Current.” The INA156’s low output impedance at high

frequencies makes it suitable for directly driving Capaci-

tive Digital-to-Analog (CDAC) input A/D converters, as

shown in Figure 9.

INPUT PROTECTION

Device inputs are protected by ESD diodes that will conduct

if the input voltages exceed the power supplies by more than

500mV. Momentary voltages greater than 500mV beyond

the power supply can be tolerated if the current on the input

pins is limited to 10mA. This is easily accomplished with

input resistors RLIM, as shown in Figure 8. Many input

signals are inherently current-limited to less than 10mA.

Therefore, a limiting resistor is not required.

INA156

7

6

4

5

Ref

I

L

2.5A

G = 10

Pins 1 and 8 Open

V+

NOTE: Output is referred to V+.

2

3

1

8

0.02

Ω

Load

50mV

+5V

INA156

ADS7818

or

ADS7834

12-Bits

f

SAMPLE = 500kHz

6

7

4

5

3

2

NOTE: G = 10 configuration

1

8

INA156

5

V

OUT

V

REF

R

LIM

R

LIM

I

OVERLOAD

10mA max

3

6

2

1

8

Input Common-Mode Voltage (V)

0.0

0.5

1.0

2.5

3.0

1.5

2.0

5

4

3

2

1

0

–1

–2

–3

–4

–5

3.5

4.0

4.5

5.5

5.0

V

S = 5.5V

P-Channel Operation

Transistion

Region

N-Channel

Operation