Texas Instruments SoundPlus INA1651EVM, Руководство пользователя, Страница: 5 / 20

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Time Constants (
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Modifications
5 SBOU212 – September 2018
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Copyright © 2018, Texas Instruments Incorporated
INA1651EVM SoundPlus™ High Common-Mode Rejection Line Receiver
Evaluation Module
3.3 Start-Up Time
Capacitor C10 increases the settling time of the VMID (OUT) pin due to the resistance of the internal
voltage divider for VMID (IN) and C10 creating an RC time constant.
Equation 5 calculates one RC time
constant.
(5)
Figure 3 displays the rate at which an RC circuit charges to its final value. Notice at one time constant, the
voltage across the capacitor is at 63.2% of its final value.
Figure 3. RC Charging Rate
Table 2 shows the final capacitor voltage at multiple time constants.
Table 2. Final Capacitor Voltage
Time Constant ( τ
RC
) Final Capacitor Voltage (%)
1 63.2%
2 86.4%
3 95.0%
4 98.1%
5 99.3%
6 99.7%
To decrease the settling time of the VMID (OUT) voltage the Zener diode, D1, can be populated. The
Zener voltage of the diode should be chosen to be greater than 100 mV more than one half of the supply
voltage.
Equation 6 calculates the recommended Zener voltage of the diode (Vz). Note that due to the
leakage current of the Zener diode interacting with the internal voltage divider resistance an error may be
seen on the VMID (OUT) pin.
(6)
3.4 Increasing the Common-Mode Input Impedance
The high CMRR of many line receivers can be degraded by source impedance mismatches in the system.
Resistor R4 provides an option to increase the common-mode input impedance of the INA1651 to reduce
the effects of source impedance mismatch. For an explanation on the effects of increasing the common-
mode input impedance, refer to section 8.1.2 of the INA1651 product data sheet (
SBOS818 ).