Datasheet

AD7776/AD7777/AD7778
–5–
REV. A
PIN FUNCTION DESCRIPTION
Mnemonic Description
V
CC
+5 V Power Supply.
AGND Analog Ground.
DGND Digital Ground. Ground reference for digital circuitry.
DB0–DB9 Input/Output Data Bus. This is a bidirectional data port from which ADC output data may be read and to which
control register data may be written.
BUSY/INT Busy/Interrupt Output. Active low logic output indicating A/D converter status. This logic output has two modes
of operation depending on whether location CR9 of the control register has been set low or high:
If CR9 is set low, the BUSY/INT output behaves as a BUSY signal. The BUSY signal goes low and stays low for the
duration of a single conversion, or if simultaneous sampling has been selected, BUSY stays low for the duration of
both conversions.
If CR9 is set high, BUSY/INT output behaves as an INTERRUPT signal. The INT signal goes low and remains low
after either a single conversion is completed or after a double conversion is completed if simultaneous sampling has
been selected. With CR9 high, the falling edge of WR or RD resets the INT line high.
CS Chip Select Input. The device is selected when this input is low.
WR Write Input (Active Low). It is used in conjunction with CS to write data to the control register. Data is latched to the
registers on the rising edge of WR. Following the rising edge of WR, the analog input is acquired and a conversion is
started.
RD Read Input (Active Low). It is used in conjunction with CS to enable the data outputs from the ADC registers.
A
IN
1–8 Analog Inputs 1–8. The analog input range is V
BIAS
± V
SWING
where V
BIAS
and V
SWING
are defined by the reference
voltage applied to REFIN. Input resistance between any of the analog input pins and AGND is 10 kΩ or greater.
REFIN Voltage Reference Input. The AD7776/AD7777/AD7778 are specified over a voltage reference range of 1.9 V to 2.1 V
with a nominal value of 2.0 V. This REFIN voltage provides the V
BIAS
and V
SWING
levels for the input channel(s).
V
BIAS
is equal to REFIN and V
SWING
is nominally equal to REFIN/2. Input resistance between this REFIN pin and
AGND is 10 kΩ or greater.
REFOUT Voltage Reference Output. This pin provides the internal voltage reference, which is nominally 2.0 V. It can provide
the bias voltage (V
BIAS
) for the input channel(s).
C
REFIN
Reference Decoupling Capacitor. A 10 nF capacitor must be connected from this pin to AGND to ensure correct
operation of the high speed ADC.
RTN Signal Return Path for the input channel(s). Normally RTN is connected to AGND at the package.
CIRCUIT DESCRIPTION
ADC Transfer Function
For all versions, an input signal of the form V
BIAS
± V
SWING
is
expected. This V
BIAS
signal level operates as a pseudo ground to
which all input signals must be referred. The V
BIAS
level is
determined by the voltage applied to the REFIN pin. This can
be driven by an external voltage source or, alternatively, by the
onboard 2 V reference, available at REFOUT. The magnitude
of the input signal swing is equal to V
BIAS
/2 (or REFIN/2) and is
set internally. With a REFIN of 2 V, the analog input signal level
varies from 1 V to 3 V, i.e., 2 ± 1 V. Figure 5 shows the transfer
function of the ADC and its relationship to V
BIAS
and V
SWING
.
The half-scale two's complement code of the ADC, 000 Hex (00
0000 0000 Binary), occurs at an input voltage equal to V
BIAS
. The
input full-scale range of the ADC is equal to 2 V
SWING
, so that the
Plus Full-Scale transition (1FE to 1FF) occurs at a voltage equal to
V
BIAS
+ V
SWING
– 1.5 LSBs, and the minus full-scale code transi-
tion (200 to 201) occurs at a voltage V
BIAS
– V
SWING
+ 0.5 LSBs.
ADC
OUTPUT
CODE
(HEX)
1FF
1FE
202
201
200
000
V
BIAS
–V
SWING
V
BIAS
V
BIAS
+V
SWING
ANALOG INPUT, V
IN
Figure 5. ADC Transfer Function