S1
S1
C =3pF
INP
C =3pF
INN
VINN
VINP
VDD1
GND1
GND1
GND1
3pF
R =28kW
IN
3pF
VINN
VINP
Equivalent
Circuit
R =
IN
f C·
CLK DIFF
1
( =10MHz)f
CLK
C =3.6pF
IND
GND1
GND1
400W
400W
S2
S2
AGND+0.8V
AGND+0.8V
AMC1200
www.ti.com
SBAS542 –APRIL 2011
THEORY OF OPERATION
INTRODUCTION
The differential analog input of the AMC1200 is a switched-capacitor circuit based on a second-order modulator
stage that digitizes the input signal into a 1-bit output stream. The device compares the differential input signal
(V
IN
= VINP – VINN) against the internal reference of 2.5 V using internal capacitors that are continuously
charged and discharged with a typical frequency of 10 MHz. With the S1 switches closed, C
IND
charges to the
voltage difference across VINP and VINN. For the discharge phase, both S1 switches open first and then both
S2 switches close. C
IND
discharges to approximately AGND + 0.8V during this phase. Figure 31 shows the
simplified equivalent input circuitry.
Figure 31. Equivalent Input Circuit
The analog input range is tailored to directly accommodate a voltage drop across a shunt resistor used for
current sensing. However, there are two restrictions on the analog input signals, VINP and VINN. If the input
voltage exceeds the range AGND – 0.5 V to AVDD + 0.5 V, the input current must be limited to 10 mA to prevent
the implemented input protection diodes from damage. In addition, the linearity and the noise performance of the
device are ensured only when the differential analog input voltage remains within ±250 mV.
The isolated digital bit stream is processed by a third-order analog filter on the low-side and presented as a
differential output of the device.
The SiO
2
-based capacitive isolation barrier supports a high level of magnetic field immunity, as described in
application report SLLA181, ISO72x Digital Isolator Magnetic-Field Immunity (available for download at
www.ti.com).
Copyright © 2011, Texas Instruments Incorporated 11
