Broadcom ACPL-C740 User Manual

ACPL-C740 Evaluation Kit Board
Isolated Sigma-Delta Modulator
Description
The Broadcom ACPL-C740 isolated sigma-delta (Σ-Δ) modulator converts an analog input signal into a high-speed (20 MHz
typical) single-bit data stream by means of a sigma-delta over-sampling modulator. The time average of the modulator data
is directly proportional to the input signal voltage. The modulator uses an internal speed of 20 MHz. The modulator data are
encoded and transmitted across the isolation boundary where they are recovered and decoded into a high-speed data
stream of digital ones and zeros. The original signal information is represented by the density of ones in the data output.
The input signal information is contained in the modulator output data stream, represented by the density of ones and zeros.
The density of ones is proportional to the input signal voltage, as shown in
produces a data stream of ones 50% of the time and zeros 50% of the time. A differential input of –200 mV corresponds to
an 18.75% density of ones, and a differential input of +200 mV is represented by an 81.25% density of ones in the data
stream. A differential input of +320 mV or higher results in ideally all ones in the data stream, while an input of –320 mV or
lower results in all zeros, ideally.
Figure 1: Modulator Output vs. Analog Input
MODULATOR OUTPUT
ANALOG INPUT
Table 1: Input Voltage with Ideal Corresponding Density of 1s at Module Data Output, and ADC Code
Analog Input
+Full-Scale
+Recommended Input Range
Zero
–Recommended Input Range
–Full-Scale
Broadcom
Table 1 shows this relationship.
Voltage Input Density of 1s
+320 mV 100%
+200 mV 81.25%
0 mV 50%
–200 mV 18.75%
–320 mV 0%
Figure 1. A differential input signal of 0V ideally
+FS (ANALOG INPUT)
0V (ANALOG INPUT)
–FS (ANALOG INPUT)
TIME
Density of 0s ADC Code (16-bit unsigned decimation)
0% 65,535
18.75% 53,248
50% 32,768
81.25% 12,288
100% 0
User Guide
ACPL-C740-EvalKit-UG100
January 4, 2019