AMC1304L05_15 Datasheet, PDF (20/39 Pages) Texas Instruments – AMC1304x High-Precision,Reinforced Isolated Delta-Sigma Modulators with LDO

AMC1304L05, AMC1304L25, AMC1304M05, AMC1304M25
SBAS655C – SEPTEMBER 2014 – REVISED SEPTEMBER 2015
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Feature Description (continued)
8.3.2 Modulator
The modulator implemented in the AMC1304 is a second-order, switched-capacitor, feed-forward ΔΣ modulator,
such as the one conceptualized in Figure 45. The analog input voltage V_IN and the output V₅ of the 1-bit digital-
to-analog converter (DAC) are differentiated, providing an analog voltage V₁ at the input of the first integrator
stage. The output of the first integrator feeds the input of the second integrator stage, resulting in output voltage
V₃ that is differentiated with the input signal V_IN and the output of the first integrator V₂. Depending on the polarity
of the resulting voltage V₄, the output of the comparator is changed. In this case, the 1-bit DAC responds on the
next clock pulse by changing its analog output voltage V₅, causing the integrators to progress in the opposite
direction and forcing the value of the integrator output to track the average value of the input.
f_CLKIN
V₁
V₂
V₃
V₄
V_IN
Integrator 1
Integrator 2
CMP
0V
V₅
DAC
Figure 45. Block Diagram of a Second-Order Modulator
The modulator shifts the quantization noise to high frequencies, as shown in Figure 46. Therefore, use a low-
pass digital filter at the output of the device to increase the overall performance. This filter is also used to convert
from the 1-bit data stream at a high sampling rate into a higher-bit data word at a lower rate (decimation). TI's
microcontroller families TMS320F2807x and TMS320F2837x offer a suitable programmable, hardwired filter
structure termed a sigma-delta filter module (SDFM) optimized for usage with the AMC1304 family. Also,
SD24_B converters on the MSP430F677x microcontrollers offer a path to directly access the integrated sinc-
filters, thus offering a system-level solution for multichannel, isolated current sensing. An additional option is to
use a suitable application-specific device, such as the AMC1210 (a four-channel digital sinc-filter). Alternatively, a
field-programmable gate array (FPGA) can be used to implement the filter.
0
-20
-40
-60
-80
-100
-120
-140
10
100
1k
10k 100k
1G
10G
Frequency (Hz)
Figure 46. Quantization Noise Shaping
20
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