AMC1305L25 Datasheet, PDF (25/39 Pages) Texas Instruments – AMC1305x High-Precision, Reinforced Isolated Delta-Sigma Modulators

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AMC1305L25, AMC1305M05, AMC1305M25
SBAS654C – JUNE 2014 – REVISED DECEMBER 2014
Typical Applications (continued)
10.2.2 Isolated Voltage Sensing
The AMC1305 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 impact of the (usually higher) impedance
of the resistor used in this case is considered.
High voltage
potential
5V
R1
AVDD
R2
AMC1305
AINP
R4
R5
IIB
-
R3
RID
+
AINN
û-Modulator
R3'
R4'
R5'
AGND
VCM = 2 V
GND
Figure 53. Using AMC1305 for Isolated Voltage Sensing
10.2.2.1 Design Requirements
Figure 53 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 AMC1305. This resistor and the
differential input impedance of the device (the AMC1305x25 is 25 kΩ, the AMC1305M05 is 5 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 AMC1305.
E _Gtot
E_G
 ^R3
R _IN
(4)
This gain error can be easily minimized during the initial system level gain calibration procedure.
10.2.2.2 Detailed Design Procedure
As indicated in Figure 53, the output of the integrated differential amplifier is internally biased to a common-mode
voltage of 2 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. Because the value of this bias current depends on the actual common-mode amplitude of the input signal (as
shown in Figure 54), the initial system offset calibration does not minimize its effect. Therefore, in systems with
high accuracy requirements TI recommends using a series resistor at the negative input (AINN) of the AMC1305
with a value equal to the shunt resistor R3 (that is R3' = R3 in Figure 53) to eliminate the effect of the bias
current.
Copyright © 2014, Texas Instruments Incorporated
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