AMC1306E05_17 Datasheet, PDF (27/40 Pages) Texas Instruments – Small, High-Precision, Reinforced Isolated Delta-Sigma Modulators with High CMTI

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AMC1306E05, AMC1306E25, AMC1306M05, AMC1306M25
SBAS734 – MARCH 2017
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.
High Voltage
Potential
R1
3.3 V
or 5 V
AVDD
R2
AINP
R4
R5
IIB
R3
RIND
AINN
û Modulator
R3'
R4'
R5'
AGND
AGND
VCM = 1.9 V
Figure 54. Using the AMC1306 for Isolated Voltage Sensing
9.2.2.1 Design Requirements
Figure 54 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 R_IN having a
considerably higher value than R3, the resulting total gain error can be estimated using Equation 4, with E_G
being the gain error of the AMC1306.
E_Gtot
= E_G
+ ^R3
R_IN
(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 54, 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 I_IB 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 55). 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 54) to eliminate
the effect of the bias current.
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