AMC1305L25 Datasheet, PDF (18/39 Pages) Texas Instruments – AMC1305x High-Precision, Reinforced Isolated Delta-Sigma Modulators

AMC1305L25, AMC1305M05, AMC1305M25
SBAS654C – JUNE 2014 – REVISED DECEMBER 2014
9 Detailed Description
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9.1 Overview
The differential analog input (AINP and AINN) of the AMC1305 is a fully-differential amplifier feeding the
switched-capacitor input of a second-order delta-sigma (ΔΣ) modulator stage that digitizes the input signal into a
1-bit output stream. The isolated data output (DOUT) of the converter provides a stream of digital ones and zeros
synchronous to the eternally-provided clock source at the CLKIN pin with a frequency in the range of 5 MHz to
20.1 MHz. The time average of this serial bit-stream output is proportional to the analog input voltage.
The Functional Block Diagram section shows a detailed block diagram of the AMC1305. The analog input range
is tailored to directly accommodate a voltage drop across a shunt resistor used for current sensing. The SiO₂-
based capacitive isolation barrier supports a high level of magnetic field immunity as described in the application
report ISO72x Digital Isolator Magnetic-Field Immunity (SLLA181A), available for download at www.ti.com. The
external clock input simplifies the synchronization of multiple current-sense channels on the system level. The
extended frequency range of up to 20 MHz supports higher performance levels compared to other solutions
available on the market.
9.2 Functional Block Diagram
AVDD
DVDD
AINP
AINN
BUF
TX
-
û-Modulator
+
TX
BUF
DOUT
DOUT_N (AMC1305L25 only)
1.25-V
Reference
BUF
AMC1305
TX
CLKIN
CLKIN_N (AMC1305L25 only)
TX
AGND
DGND
9.3 Feature Description
9.3.1 Analog Input
The AMC1305 incorporates front-end circuitry that contains a differential amplifier and sampling stage, followed
by a ΔΣ modulator. The gain of the differential amplifier is set by internal precision resistors to a factor of 4 for
devices with a specified input voltage range of ±250 mV (for the AMC1305x25), or to a factor of 20 for devices
with a ±50-mV input voltage range (for the AMC1305M05), resulting in a differential input impedance of 5 kΩ (for
the AMC1305M05) or 25 kΩ (for the AMC1305x25).
Consider the input impedance of the AMC1305 in designs with high-impedance signal sources that can cause
degradation of gain and offset specifications. The importance of this effect, however, depends on the desired
system performance. Additionally, the input bias current caused by the internal common-mode voltage at the
output of the differential amplifier causes an offset that depends on the actual amplitude of the input signal. See
the Isolated Voltage Sensing section for more details on reducing these effects.
There are two restrictions on the analog input signals (AINP and AINN). First, if the input voltage exceeds the
range of AGND – 6 V to AVDD + 0.5 V, the input current must be limited to 10 mA because the device input
electrostatic discharge (ESD) protection diodes turn on. In addition, the linearity and noise performance of the
device are ensured only when the differential analog input voltage remains within the specified linear full-scale
range (FSR), that is ±250 mV (for the AMC1305x25) or ±50 mV (for the AMC1305M05), and within the specified
input common-mode range.
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