AMC1303E0510 Datasheet, PDF (28/40 Pages) Texas Instruments – Small, High-Precision, Reinforced Isolated Delta-Sigma Modulators with Internal Clock

AMC1303E0510, AMC1303M0510, AMC1303E0520, AMC1303M0520
AMC1303E2510, AMC1303M2510, AMC1303E2520, AMC1303M2520
SBAS771 – JUNE 2017
www.ti.com
9.2.2.2 Detailed Design Procedure
As indicated in Figure 50, 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. Because the value of this bias current depends on the actual common-mode amplitude of the input signal (as
illustrated in Figure 51), 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
AMC1303 with a value equal to the shunt resistor R3 (that is, R3' = R3 in Figure 50) 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 5 with R5 = R5' = 50 kΩ and R4 = R4' = 12.5 kΩ for the AMC1303x25x.
E _G(%)
§
¨©¹
R4
R4' R3
'
·
¸¹
u
100%
(5)
9.2.2.3 Application Curve
Figure 51 shows the dependency of the input bias current on the common-mode voltage at the input of the
AMC1303x25x.
60
40
20
0
-20
-40
-60
-80
-0.5 0 0.5 1 1.5 2 2.5 3 3.5
V_CM (V)
D005
AMC1303x25x
Figure 51. Input Current vs Input Common-Mode Voltage
9.2.3 Do's and Don'ts
Do not leave the inputs of the AMC1303 unconnected (floating) when the device is powered up. If both modulator
inputs are left floating, the input bias current drives these inputs to the output common-mode voltage of the
differential amplifier of approximately 1.9 V. If that voltage is above the specified input common-mode range, the
gain of the differential amplifier diminishes and the modulator outputs a bitstream resembling a zero differential
input voltage.
28
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