LTC1272
19
1272fc
For more information www.linear.com/1272
APPLICATIONS INFORMATION
If the clock signal for the AD7572 is derived from a separate
crystal or other signal which is not synchronous with the
microprocessor clock, then the signals need to be synchro-
nized for the LTC1272 to achieve best analog performance
(see Clock and Control Synchronization). The best way
to synchronize these signals is to drive the CLK IN pin of
the LTC1272 with a derivative of the processor clock, as
mentioned above and shown in Figure 22. Another way,
shown in Figure 23, is to use a flip-flop to synchronize the
RD to the LTC1272 with the CLK IN signal. This method
will work but has two disavantages over the first: because
the RD is delayed by the flip-flop, the actual conversion
start and the enabling of the LTC1272’s BUSY and data
outputs can take up to one CLK IN cycle to respond to a
RD↓ convert command from the processor. The sampling
of the analog input no longer occurs at the processor’s
falling RD edge but may be delayed as much as one CLK
IN cycle. Although the LTC1272 will still exhibit excellent
DC performance, the flip-flop will introduce jitter into the
sampling which may reduce the usefulness of this method
for AC systems.
Figure 22. Plugging the LTC1272 into an AD7572 Socket
Case 1: Clock Synchronous with CS and RD
10µF
IN
REF
D11 (MSB)
D10
D9
D8
D7
CLK IN**
CLK OUT
HBEN
RD
CS
BUSY
NC
V
LTC1272
D6
D5
D4
DGND
D3/11
D2/10
D1/9
D0/8
A
V
AGND
DD
✝
µP
CONTROL
LINES
0.1µF
–15V
µP
DATA
BUS
ANALOG INPUT
(0V TO 5V)
10µF
0.1µF
THE LTC1272 HAS THE SAME 0V TO 5V INPUT RANGE BUT PROVIDES A 2.42V
REFERENCE OUTPUT AS OPPOSED TO THE –5.25V OF THE AD7572. FOR PROPER
OPERATION, REVERSE THE REFERENCE CAPACITOR POLARITY AND SHORT OUT THE
10Ω RESISTOR.
*
THE ADC CLOCK SHOULD BE SYNCHRONIZED TO THE CONVERSION START
SIGNALS (CS, RD) OR 1-2 LSBs OF OUTPUT CODE NOISE MAY OCCUR. DERIVING
THE ADC CLOCK FROM THE µP CLOCK IS ADEQUATE.
**
THE LTC1272 CAN ACCOMMODATE THE –15V SUPPLY OF THE AD7572 BUT DOES
NOT REQUIRE IT. PIN 23 OF THE LTC1272 IS NOT INTERNALLY CONNECTED.
LTC1272 • F22
2.42V
V
OUTPUT
*
REF
10Ω*
+
10µF
0.1µF
5V
+
✝
+
†
