Datasheet
AD7785
Rev. 0 | Page 20 of 32
ADC CIRCUIT INFORMATION
OVERVIEW
The AD7785 is a low power ADC that incorporates a ∑-Δ
modulator, a buffer, reference, in-amp, and an on-chip digital
filter intended for the measurement of wide dynamic range, low
frequency signals such as those in pressure transducers, weigh
scales, and temperature measurement applications.
The part has three differential inputs that can be buffered or
unbuffered. The device can be operated with the internal 1.17 V
reference, or an external reference can be used.
Figure 12 shows
the basic connections required to operate the part.
The output rate of the AD7785 (f
ADC
) is user-programmable. The
allowable update rates, along with their corresponding settling
times, are listed in
Tabl e 14. Normal mode rejection is the major
function of the digital filter. Simultaneous 50 Hz and 60 Hz
rejection is optimized when the update rate equals 16.7 Hz or
less as notches are placed at both 50 Hz and 60 Hz with these
update rates (see
Figure 14.)
The AD7785 uses slightly different filter types, depending on
the output update rate so that the rejection of quantization
noise and device noise is optimized. When the update rate is
from 4.17 Hz to 12.5 Hz, a Sinc3 filter, along with an averaging
filter, is used. When the update rate is from 16.7 Hz to 39 Hz, a
modified Sinc3 filter is used. This filter provides simultaneous
50 Hz/60 Hz rejection when the update rate equals 16.7 Hz. A
Sinc4 filter is used when the update rate is from 50 Hz to
242 Hz. Finally, an integrate-only filter is used when the update
rate equals 470 Hz.
Figure 13 to Figure 16 show the frequency response of the
different filter types for several update rates.
06721-012
DOUT/RD
Y
DIN
SCLK
CS
DV
DD
SERIAL
INTERFACE
AND
CONTROL
LOGIC
Σ-Δ
ADC
AD7785
AIN2(+)
REFIN(+)
REFIN(–)
AIN2(–)
AV
DD
GND
MUX
BAND GAP
REFERENCE
INTERNAL
CLOCK
CLK
GND
GND
A
V
DD
AV
DD
IN-AMPBUF
REFIN(+) REFIN(–)
V
BIAS
AIN1(+)
AIN1(–)
R
R
T
HERMOCOUPLE
JUNCTION
C
R
REF
IOUT2
Figure 12. Basic Connection Diagram
0
–20
–40
–60
–80
–100
0 12010080604020
06721-013
FREQUENCY (Hz)
(dB)
Figure 13. Filter Profile with Update Rate = 4.17 Hz