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ACPL-7970

Optically Isolated Sigma-Delta Modulator

Avago Technologies 

Definitions

Integral Nonlinearity (INL)

INL is the maximum deviation of a transfer curve from a

straight line passing through the endpoints of the ADC

transfer function, with offset and gain errors adjusted

out.

Differential Nonlinearity (DNL)

DNL is the deviation of an actual code width from the ideal

value of 1 LSB between any two adjacent codes in the ADC

transfer curve. DNL is a critical specification in closed-loop

applications. A DNL error of less than ±1 LSB guarantees

no missing codes and a monotonic transfer function.

Offset Error

Offset error is the deviation of the actual input voltage

corresponding to the mid-scale code (32,768 for a 16-bit

system with an unsigned decimation filter) from 0 V. Offset

error can be corrected by software or hardware.

Gain Error (Full-Scale Error)

Gain error includes positive full-scale gain error and

negative full-scale gain error. Positive full-scale gain error

is the deviation of the actual input voltage correspond-

ing to positive full-scale code (65,535 for a 16-bit system)

from the ideal differential input voltage (VIN+ – VIN– =

+320 mV), with offset error adjusted out. Negative full-

scale gain error is the deviation of the actual input voltage

corresponding to negative full-scale code (0 for a 16-bit

system) from the ideal differential input voltage (VIN+ –

VIN– = -320 mV), with offset error adjusted out. Gain error

includes reference error. Gain error can be corrected by

software or hardware.

Signal-to-Noise Ratio (SNR)

The SNR is the measured ratio of AC signal power to noise

power below half of the sampling frequency. The noise

power excludes harmonic signals and DC.

Signal-to-(Noise + Distortion) Ratio (SNDR)

The SNDR is the measured ratio of AC signal power to

noise plus distortion power at the output of the ADC. The

signal power is the rms amplitude of the fundamental

input signal. Noise plus distortion power is the rms sum

of all non-fundamental signals up to half the sampling

frequency (excluding DC).

Effective Number of Bits (ENOB)

The ENOB determines the effective resolution of an ADC,

expressed in bits, defined by ENOB = (SNDR − 1.76)/6.02

Isolation Transient Immunity (CMR)

The isolation transient immunity (also known as Common-

Mode Rejection or CMR) specifies the minimum rate-of-

rise/fall of a common-mode signal applied across the

isolation boundary beyond which the modulator clock or

data is corrupted.

Product Overview

Description

The ACPL-7970 isolated sigma-delta (∑-∆) modulator

converts an analog input signal into a high-speed (10

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 internal clock of 10 MHz. The

modulator data are encoded and transmitted across the

isolation boundary where they are recovered and decoded

into 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 other main function of the modulator (optocoupler)

is to provide galvanic isolation between the analog signal

input and the digital data output. It provides high noise

margins and excellent immunity against isolation-mode

transients that allows direct measurement of low-level

signals in highly noisy environments, for example mea-

surement of motor phase currents in power inverters.

With 0.5 mm minimum DTI, the ACPL-7970 provides

reliable double protection and high working insulation

voltage, which is suitable for fail-safe designs. This out-

standing isolation performance is superior to alternatives

including devices based on capacitive- or magnetic-

coupling with DTI in micro-meter range. Offered in an

DIP-8 package, the isolated ADC delivers the reliability,

small size, superior isolation and over-temperature perfor-

mance motor drive designers need to accurately measure

current at much lower price compared to traditional

current transducers.

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