acpl-c797 Avago Technologies, acpl-c797 Datasheet - Page 10

• Manufacturer Part Number: acpl-c797
• Description: Optically Isolated Sigma-delta Modulator
• Manufacturer: Avago Technologies

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-C797 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-C797 provides
reliable double protection and high working insulation
voltage, which is suitable for fail-safe designs. This out-
standing isolation performance is superior to alterna-
tives including devices based on capacitive- or magnetic-
coupling with DTI in micro-meter range. Offered in an
SSO-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.
10
Analog Input
The differential analog inputs of the ACPL-C797 are im-
plemented with a fully-differential, switched-capacitor
circuit. The ACPL-C797 accepts signal of ±200 mV (full
scale ±320 mV), which is ideal for direct connection to
shunt based current sensing or other low-level signal
sources applications such as motor phase current mea-
surement. An internal voltage reference determines the
full-scale analog input range of the modulator (±320 mV);
an input range of ±200 mV is recommended to achieve
optimal performance. Users are able to use higher input
range, for example ±250 mV, as long as within full-scale
range, for purpose of over-current or overload detection.
Figure 14 shows the simplified equivalent circuit of the
analog input.
In the typical application circuit (Figure 19), the ACPL-C797
is connected in a single-ended input mode. Given the
fully differential input structure, a differential input con-
nection method (balanced input mode as shown in Figure
15) is recommended to achieve better performance. The
input currents created by the switching actions on both of
the pins are balanced on the filter resistors and cancelled
out each other. Any noise induced on one pin will be
coupled to the other pin by the capacitor C and creates
only common mode noise which is rejected by the device.
Typical values for Ra (= Rb) and C are 2.2 : and 100 nF
respectively.
V
V
Figure 14. Analog input equivalent circuit
+Analog Input
Figure 15. Simplified differential input connection diagram
–Analog Input
IN +
IN –
f
= MCLK
f
= MCLK
SWITCH
SWITCH
Ra
Rb
1.5 pF
1.5 pF
200
200
ANALOG
GROUND
:
:
(TYP)
(TYP)
C
5 V
COMMON MODE
V
V
V
GND1
DD1
IN+
IN –
VOLTAGE
ACPL-C797
3 pF (TYP)
3 pF (TYP)