AD976/AD976A
t12
t12
R/C
CS
BYTE
HI-Z
HI-Z
HI-Z
HI-Z
PINS 6–13
HIGH BYTE
LOW BYTE
LOW BYTE
HIGH BYTE
t14
t14
t9
PINS 15–22
Figure 4. Using CS and BYTE to Control Data Bus Read Timing
Regardless of the method for controlling conversions, output
R1
200⍀
data from conversion “n–1” will be valid during the BUSY Low
time for roughly 3.7 µs (AD976A only), and output data from
conversion “n” will be valid at the end of a conversion, 50 ns
(t10) before BUSY returns High. It is recommended, however,
that data is read only after BUSY goes high since this timing is
much more clearly defined and provides optimal performance.
Figure 4 demonstrates the functionality of the BYTE pin and
shows how the data will be valid in Binary Twos Complement
format only when R/C is asserted High and CS is Low. The
BYTE pin enables the output data on the bus to be read as a
full parallel output or as two 8-bit bytes on Pins 6–13 and Pins
15–22.
V
؎10V INPUT
IN
AGND1
AD976
REF
R2
33.2k⍀
C1
2.2F
CAP
C2
2.2F
AGND2
Figure 5. ±10 V Input Connection for the AD976 (Internal
Reference)
ANALOG INPUTS
Figure 5 shows the analog input section for the AD976 when
operating with an internal reference. The analog input range is
nominally a bipolar –10 V to +10 V. Since the AD976/AD976A
can be operated with an internal or external reference, the full-
R1
200⍀
؎10V INPUT
V
IN
AGND1
R2
66.4k⍀
scale analog input range can be best represented as ±4 VREF
The nominal input impedance is 23 kΩ/13 kΩ with a 22 pF
.
AD976A
C1
2.2F
REF
input capacitance. The analog input section also has a ±25 V
overvoltage protection. Since the AD976/AD976A has two
analog grounds it is important to ensure that the analog input is
referenced to the AGND1 pin, the low current ground. This
will minimize any problems associated with a resistive ground
drop. It is also important to ensure that the analog input of the
AD976/AD976A is driven by a low impedance source. With its
primarily resistive analog input circuitry, the ADC can be driven
by a wide selection of general purpose amplifiers.
V
+5V
ANA
CAP
C2
2.2F
AGND2
Figure 6. ±10 V Input Connection for the AD976A (Internal
Reference) Only
To best match the low distortion requirements of the AD976/
AD976A, care should be taken in the selection of the drive
circuitry op amp. Figure 6 shows the analog input section for
the AD976A when operating with an internal reference only.
Figure 9 shows the analog input section for both the AD976 and
the AD976A when operating with an external reference.
REV. C
–9–
ADI [ ADI ]
