AMC1204-Q1_15 Datasheet, PDF (17/33 Pages) Texas Instruments – 20 MHz, Second-Order, Isolated Delta-Sigma Modulator for Current-Shunt Measurement

AMC1204-Q1
www.ti.com
SLAS886B – JULY 2012 – REVISED JANUARY 2013
MODULATOR
The modulator topology of the AMC1204-Q1 is fundamentally a second-order, switched-capacitor, ΔΣ modulator,
such as the one conceptualized in Figure 43. The analog input voltage (X_(t)) and the output of the 1-bit digital-to-
analog converter (DAC) are differentiated, providing an analog voltage (X₂) at the input of the first integrator or
modulator stage. The output of the first integrator is further differentiated with the DAC output; the resulting
voltage (X₃) feeds the input of the second integrator stage. When the value of the integrated signal (X₄) at the
output of the second stage equals the comparator reference voltage, the output of the comparator switches from
high to low, or vice versa, depending on its previous state. In this case, the 1-bit DAC responds on the next clock
pulse by changing its analog output voltage (X₆), causing the integrators to progress in the opposite direction,
while forcing the value of the integrator output to track the average of the input.
X_(t)
X₂
Integrator 1
f_S
X₃
X₄
Integrator 2
V_REF
f_CLK
DATA
Comparator
X₆
DAC
Figure 43. Block Diagram of a Second-Order Modulator
The modulator shifts the quantization noise to high frequencies, as shown in Figure 44; therefore, a low-pass
digital filter should be used 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). A digital signal processor (DSP), microcontroller (µC), or field programmable gate array (FPGA)
can be used to implement the filter. Another option is to use a suitable application-specific device such as the
AMC1210, a four-channel digital sinc-filter.
0
-20
-40
-60
-80
-100
-120
-140
10
100
1k
10k 100k
1G
10G
Frequency (Hz)
Figure 44. Quantization Noise Shaping
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