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HCPL-7860

Optically Isolated Sigma-Delta (Σ−∆) Modulator

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Product Description

The HCPL-7860/HCPL-786J

Isolated Modulator

(optocoupler) uses sigma-delta

modulation to convert an

analog input signal into a

high-speed (10 MHz) single-bit

digital data stream; the time

average of the modulator’s

single-bit data is directly

proportional to the input

signal. The isolated

modulator’s other main

function is to provide galvanic

isolation between the analog

input and the digital output.

An internal voltage reference

determines the full-scale

analog input range of the

modulator (approximately ±

320 mV); an input range of ±

200 mV is recommended to

achieve optimal performance.

HCPL-7860/HCPL-786J can be

used together with HCPL-0872,

Digital Interface IC or a digital

filter. The primary functions of

the HCPL-0872 Digital

Interface IC are to derive a

multi-bit output signal by

averaging the single-bit

modulator data, as well as to

provide a direct

microcontroller interface. The

effective resolution of the

multi-bit output signal is a

function of the length of time

(measured in modulator clock

cycles) over which the average

is taken; averaging over longer

periods of time results in

higher resolution. The Digital

Interface IC can be configured

for five conversion modes,

which have different

combinations of speed and

resolution to achieve the

desired level of performance.

Other functions of the HCPL-

0872 Digital Interface IC

include a Phase Locked Loop

based pre-trigger circuit that

can either give more precise

Table 1. Input Full-Scale Range and Code Assignment.

control of the effective

sampling time or reduce

conversion time to less than 1

ìs, a fast over-range detection

circuit that rapidly indicates

when the magnitude of the

input signal is beyond full-

scale, an adjustable threshold

detection circuit that indicates

when the magnitude of the

input signal is above a user

adjustable threshold level, an

offset calibration circuit, and a

second multiplexed input that

allows a second Isolated

Modulator to be used with a

single Digital Interface IC.

The digital output format of

the Isolated A/D Converter is

15 bits of unsigned binary

data. The input full-scale range

and code assignment is shown

in Table 1 below. Although the

output contains 15 bits of

data, the effective resolution is

lower and is determined by

selected conversion mode as

shown in Table 2 below.

Analog Input

Voltage Input

Digital Output

Full Scale Range

640 mV

32768 LSBs

Minimum Step Size

20 µV

1 LSB

+Full Scale

+320 mV

111111111111111

Zero

0 mV

100000000000000

-Full Scale

-320 mV

000000000000000

Table 2. Isolated A/D Converter Typical Performance Characteristics.

Conversion Mode

Signal-to- Effective

Noise Ratio Resolution

(dB)

(bits)

Conversion Time (µs)

Pre-Trigger Mode

0

1

2

1

83

13.5

205

102

2

79

12.8

103

51

3

73

11.9

39

19

0.2

4

66

10.7

20

10

5

53

8.5

10

5

Notes: Bold italic type indicates Default values.

Signal

Delay(µs)

102

51

19

10

5

Signal

Bandwidth

(kHz)

3.4

6.9

22

45

90

13

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