AMC1301-Q1 Datasheet, PDF (22/34 Pages) Texas Instruments – Precision, 250-mV Input, Delay, Reinforced Isolated Amplifier

AMC1301-Q1
SBAS792A – APRIL 2017 – REVISED APRIL 2017
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9.2.2 Isolated Voltage Sensing
The AMC1301-Q1 device is optimized for usage in current-sensing applications using low-impedance shunts.
However, the device may also be used in isolated voltage-sensing applications if the effect of the (usually higher)
impedance of the resistor divider used in this case is considered.
High Voltage
Potential
3.3 V
or 5 V
R1
VDD1
R2
AMC1301-Q1 Front-End
VINP
R4
R5
IIB
+
R3
RIN
±
VINN
R3'
R4'
R5'
û -Modulator
GND1
VCM = 2 V
Copyright © 2016, Texas Instruments Incorporated
Figure 52. Using the AMC1301-Q1 Device for Isolated Voltage Sensing
9.2.2.1 Design Requirements
Figure 52 shows a simplified circuit typically used in high-voltage sensing applications. The high-impedance
resistors (R1 and R2) dominate the current value that flows through the resistive divider. The resistance of the
sensing resistor R3 is chosen to meet the input voltage range of the AMC1301-Q1 device. This resistor and the
input impedance of the device (R_IN = 18 kΩ) also create a voltage divider that results in an additional gain error.
With the assumption of R1 and R2 having a considerably higher value than R3 and omitting R3' for the moment,
the resulting total gain error is estimated using Equation 1, with E_G being the initial gain error of the AMC1301-
Q1 device.
E_Gtot
E_G
R₃
R_IN
(1)
This gain error may be easily minimized during the initial system-level gain calibration procedure.
9.2.2.2 Detailed Design Procedure
As indicated in Figure 52, 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 of this current is specified in the Pin Configuration and
Functions section. This bias current generates additional offset and gain errors that depend 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 53), the initial system offset calibration eliminates the offset but not the gain error
component. Therefore, in systems with high accuracy requirements, a series resistor is recommended to be used
at the negative input (VINN) of the AMC1301-Q1 device with a value equal to the shunt resistor R3 (that is, R3' =
R3 in Figure 52) to eliminate the effect of the bias current.
This additional series resistor (R3') influences the gain error of the circuit. The effect is calculated using
Equation 2 with R4 = R4' = 12.5 kΩ. The effect of the internal resistors R5 = R5' cancels in this calculation.
E_G (%)
¨§ 1
©
R4 _{¸· *100%}
R4' R3' _¹
(2)
22
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