AD7780
Rev. A | Page 11 of 16
THEORY OF OPERATION
0110080604020
FILTER GAIN (dB)
INPUT SIGNAL FREQUENCY (Hz)
–1000
0
–20
–40
–60
–80
–100
20
07945-020
The AD7780 is a low power ADC that incorporates a precision
24-bit, Σ-∆ modulator; a PGA; and an on-chip digital filter
intended for measuring wide dynamic range, low frequency
signals. The part provides a complete front-end solution for
bridge sensor applications such as weigh scales and pressure
sensors.
The device has an internal clock and one buffered differential
input. It offers a choice of two update rates (10 Hz or 16.7 Hz)
and two gain settings (1 or 128). These functions are controlled
using dedicated pins, which makes the interface easy to configure.
A 2-wire interface simplifies data retrieval from the AD7780.
FILTER, DATA RATE, AND SETTLING TIME
The AD7780 has two filter options. When the FILTER pin is low,
the 16.7 Hz filter is selected; when the FILTER pin is high, the
10 Hz filter is selected. When the polarity of FILTER is changed,
the AD7780 modulator and filter are reset immediately. DOUT/
RDY
is set high, and the ADC then begins conversions using
the selected filter response. The first conversion requires the
complete settling time of the filter. Subsequent conversions occur
at the selected update rate. The settling time of the 10 Hz filter is
300 ms (three conversion cycles), and the settling time of the
16.7 Hz filter is 120 ms (two conversion cycles).
Figure 20. Filter Profile with Update Rate = 16.7 Hz (FILTER = 0)
0110080604020
FILTER GAIN (dB)
INPUT SIGNAL FREQUENCY (Hz)
–1000
0
–20
–40
–60
–80
–100
20
07945-021
When a step change occurs on the analog input, the AD7780
requires several conversion cycles to generate a valid conversion.
If the step change occurs synchronous to the conversion period, the
settling time of the AD7780 must be allowed to generate a valid
conversion. If the step change occurs asynchronous to the end
of a conversion, an extra conversion must be allowed to generate
a valid conversion. The data register is updated with all the con-
versions, but, for an accurate result, the user must allow for the
required time.
Figure 21. Filter Profile with Update Rate = 10 Hz (FILTER = 1)
Figure 20 and Figure 21 show the filter response for each filter.
The 10 Hz filter provides greater than −45 dB of rejection in the
stop band. The only external filtering required on the analog inputs
is a simple R-C filter to provide rejection at multiples of the master
clock. A 1 k resistor in series with each analog input, a 0.01 F
capacitor from each input to GND, and a 0.1 F capacitor from
AIN(+) to AIN(−) are recommended.
When the filter is changed, DOUT/
RDY
goes high and remains
high until the appropriate settling time for that filter elapses
(see ). Therefore, the user should complete any read
operations before changing the filter. Otherwise, 1s are read
back from the AD7780 because the DOUT/
Figure 5
RDY
pin is set high
following the filter change.
