AMC1204_1108 Datasheet, PDF (18/30 Pages) Texas Instruments – 20MHz, Second-Order, Isolated Delta-Sigma Modulator for Current-Shunt Measurement

AMC1204
SBAS512B – APRIL 2011 – REVISED AUGUST 2011
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DIGITAL OUTPUT
A differential input signal of 0V ideally produces a stream of ones and zeros that are high 50% of the time and
low 50% of the time. A differential input of +250mV produces a stream of ones and zeros that are high 78.1% of
the time. A differential input of –250mV produces a stream of ones and zeros that are high 21.9% of the time.
This is also the specified linear input range of the modulator with the performance as specified in this data sheet.
The range between 250mV and 320mV (absolute values) is the non-linear range of the modulator. The output of
the modulator clipps with a stream of only zeros with an input less than or equal to –320mV or with a stream of
only ones with an input greater than or equal to 320mV. The input voltage versus the output modulator signal is
shown in Figure 45.
The system clock of the AMC1204 is typically 20MHz and is provided externally at the CLKIN pin. The data are
synchronously provided at 20MHz at the DATA output pin. The data are changing at the falling edge of CLKIN;
for more details see the Timing Information section.
Modulator Output
+FS (Analog Input)
-FS (Analog Input)
Analog Input
Figure 45. Analog Input versus AMC1204 Modulator Output
FILTER USAGE
The modulator generates a bit stream that is processed by a digital filter to obtain a digital word similar to a
conversion result of a conventional analog-to-digital converter (ADC). A very simple filter, built with minimal effort
and hardware, is a sinc³-type filter, as shown in Equation 1:
3
H(z) =
1 - z^-OSR
1 - z^-1
(1)
This filter provides the best output performance at the lowest hardware size (count of digital gates). For an
oversampling rate (OSR) in the range of 16 to 256, this filter is a good choice. All the characterization in this
document is also done with a sinc³ filter with OSR = 256 and an output word width of 16 bits.
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