AMC1304L05 Datasheet, PDF (19/39 Pages) Texas Instruments – High-Precision Reinforced Isolated Delta-Sigma Modulators

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8 Detailed Description
AMC1304L05, AMC1304L25, AMC1304M05, AMC1304M25
SBAS655C – SEPTEMBER 2014 – REVISED SEPTEMBER 2015
8.1 Overview
The differential analog input (AINP and AINN) of the AMC1304 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 and DOUT_N) of the converter provides a stream of digital
ones and zeros that is synchronous to the externally-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 AMC1304. 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-sensing channels on the system level. The
extended frequency range of up to 20 MHz supports higher performance levels compared to the other solutions
available on the market.
8.2 Functional Block Diagram
LDOIN
Voltage Regulator
(LDO)
VCAP
DVDD
AINP
AINN
BUF
TX
-
û -Modulator
+
TX
BUF
DOUT
DOUT_N (AMC1304Lx only)
1.25-V
Reference
BUF
AMC1304
TX
CLKIN
CLKIN_N (AMC1304Lx only)
TX
AGND
DGND
8.3 Feature Description
8.3.1 Analog Input
The AMC1304 incorporates a 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 (this value is for the AMC1304x25), or to a factor of 20
in devices with a ±50-mV input voltage range (for the AMC1304x05), resulting in a differential input impedance of
5 kΩ (for the AMC1304x05) or 25 kΩ (for the AMC1304x25).
Consider the input impedance of the AMC1304 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 is dependent 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 AGND – 6 V to 3.7 V, the input current must be limited to 10 mA because the device input electrostatic
discharge (ESD) 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 AMC1304x25) or ±50 mV (for the AMC1304x05), and within the specified input common-mode
range.
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