Gtot

G

IN

R3

E

= E

+

R

û Modulator

VCM = 1.9 V

3.3 V

or 5 V

High Voltage

Potential

R1

R2

R3

R4

R5

R4'

R5'

IIB

RIND

AVDD

AINP

AINN

AGND

R3'

AGND

31

AMC1306E05, AMC1306E25, AMC1306M05, AMC1306M25

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SBAS734C – MARCH 2017 – REVISED JANAURY 2020

Product Folder Links: AMC1306E05 AMC1306E25 AMC1306M05 AMC1306M25

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9.2.2 Isolated Voltage Sensing

The AMC1306 is optimized for usage in current-sensing applications using low-impedance shunts. However, the

device can also be used in isolated voltage-sensing applications if the affect of the (usually higher) impedance of

the resistor used in this case is considered.

Figure 59. Using the AMC1306 for Isolated Voltage Sensing

9.2.2.1

Design Requirements

Figure 59 shows a simplified circuit typically used in high-voltage-sensing applications. The high impedance

resistors (R1 and R2) are used as voltage dividers and dominate the current value definition. The resistance of

the sensing resistor R3 is chosen to meet the input voltage range of the AMC1306. This resistor and the

differential input impedance of the device (the AMC1306x25 is 22 kΩ, the AMC1306x05 is 4.9 kΩ) also create a

voltage divider that results in an additional gain error. With the assumption of R1, R2, and RIN having a

considerably higher value than R3, the resulting total gain error can be estimated using Equation 4, with EG

being the gain error of the AMC1306.

(4)

This gain error can be easily minimized during the initial system-level gain calibration procedure.

9.2.2.2 Detailed Design Procedure

As indicated in Figure 59, the output of the integrated differential amplifier is internally biased to a common-mode

voltage of 1.9 V. This voltage results in a bias current IIB through the resistive network R4 and R5 (or R4' and

R5') used for setting the gain of the amplifier. The value range of this current is specified in the Electrical

Characteristics table. This bias current generates additional offset error that depends on the value of the resistor

R3. The initial system offset calibration does not minimize this effect because the value of the bias current

depends on the actual common-mode amplitude of the input signal (as illustrated in Figure 60). Therefore, in

systems with high accuracy requirements, a series resistor is recommended to be used at the negative input

(AINN) of the AMC1306 with a value equal to the shunt resistor R3 (that is, R3' = R3 in Figure 59) to eliminate

the effect of the bias current.