REV. A
AD974
–18–
0
–130
12
–90
–100
–110
–120
–70
–80
FREQUENCY – kHz
dBFS
4 6 8 10 12 14 16 18 20
–60
–40
–50
–30
–10
–20
Figure 24. Adjacent Channel Crosstalk, Worst Pair (8192
Point FFT; AIN 2 = 220 kHz, –0.1 dB; AIN 1 = AGND)
MICROPROCESSOR INTERFACING
The AD974 is ideally suited for traditional dc measurement
applications supporting a microprocessor, and ac signal process-
ing applications interfacing to a digital signal processor. The
AD974 is designed to interface with a general purpose serial
port or I/O ports on a microcontroller. A variety of external
buffers can be used with the AD974 to prevent digital noise
from coupling into the ADC. The following sections illustrate
the use of the AD974 with an SPI equipped microcontroller and
the ADSP-2181 signal processor.
SPI INTERFACE
Figure 25 shows a general interface diagram between the
AD974 and an SPI equipped microcontroller. This interface
assumes that the convert pulses will originate from the micro-
controller and that the AD974 will act as the slave device. The
convert pulse could be initiated in response to an internal timer
interrupt. The reading of output data, one byte at a time,
if necessary, could be initiated in response to the end-of-
conversion signal (BUSY going high).
+5V
SDI
SCK
I/O PORT
IRQ
SPI
DATA
DATACLK
R/C
BUSY
EXT/INT
CS
AD974
Figure 25. AD974-to-SPI Interface
ADSP-2181 INTERFACE
Figure 26 shows an interface between the AD974 and the
ADSP-2181 Digital Signal Processor. The AD974 is configured
for the Internal Clock mode (EXT/INT = 0) and will therefore
act as the master device. The convert command is shown gener-
ated from an external oscillator in order to provide a low jitter
signal appropriate for both dc and ac measurements. Because
the SPORT, within the ADSP-2181, will be seeing a discontinu-
ous external clock, some steps are required to ensure that the
serial port is properly synchronized to this clock during each
data read operation. The recommended procedure to ensure
this is as follows:
• Enable SPORT0 through the System Control register.
• Set the SCLK Divide register to zero.
• Setup PF0 and PF1 as outputs by setting bits 0 and 1 in
PFTYPE.
• Force RFS0 low through PF0. The Receive Frame Sync
signal has been programmed active high.
• Enable AD974 by forcing CS = 0 through PF1.
• Enable SPORT0 Receive Interrupt through the IMASK
register.
• Wait for at least one full conversion cycle of the AD974 and
throw away the received data.
• Disable the AD974 by forcing CS = 1 through PF1.
• Wait for a period of time equal to one conversion cycle.
• Force RFS0 high through PF0.
• Enable the AD974 by forcing CS = 0 through PF1.
The ADSP-2181 SPORT0 will now remain synchronized to the
external discontinuous clock for all subsequent conversions.
DR0
SCLK0
PF1
RFS0
ADSP-2181
DATA
DATACLK
R/C
EXT/INT
CS
AD974
PF0
OSCILLATOR
SPORT0 CNTRL REG = 03300F
Figure 26. AD974-to-ADSP-2181 Interface
POWER SUPPLIES AND DECOUPLING
The AD974 has two power supply input pins. V
ANA
and V
DIG
provide the supply voltages to the analog and digital portions,
respectively. V
ANA
is the +5 V supply for the on-chip analog
circuitry, and V
DIG
is the +5 V supply for the on-chip digital
circuitry. The AD974 is designed to be independent of power
supply sequencing and thus free from supply voltage induced
latchup.
With high performance linear circuits, changes in the power
supplies can result in undesired circuit performance. Optimally,
well regulated power supplies should be chosen with less than
1% ripple. The ac output impedance of a power supply is a
complex function of frequency and will generally increase with
frequency. Thus, high frequency switching, such as that en-
countered with digital circuitry, requires the fast transient cur-
rents that most power supplies cannot adequately provide. Such
a situation results in large voltage spikes on the supplies. To
compensate for the finite ac output impedance of most supplies,
charge “reserves” should be stored in bypass capacitors. This
will effectively lower the supplies impedance presented to the
AD974 V
ANA
and V
DIG
pins and reduce the magnitude of these
spikes. Decoupling capacitors, typically 0.1␣ µF, should be placed
close to the power supply pins of the AD974 to minimize any
inductance between the capacitors and the V
ANA
and V
DIG
pins.
