AD7810
–9–
REV. B
t
POWER-UP
1.5s
A
B
D
OUT
SCLK
t
1
CONVST
CURRENT CONVERSION RESULT
Figure 15. Mode 2 Operation Timing
Mode 2 Operation (Automatic Power-Down)
When used in this mode of operation, the part automatically
powers down at the end of a conversion. This is achieved by
leaving the CONVST signal low until the end of the conversion.
Because it takes approximately 1.5 µs for the part to power up
after it has been powered down, this mode of operation is in-
tended to be used in applications where slower throughput rates
are required, i.e., in the order of 100 kSPS. The timing diagram
in Figure 15 shows how to operate the part in this mode. If the
AD7810 is powered down, the rising edge of the CONVST
pulse causes the part to power up. When the part has powered
up (≈ 1.5 µs after the rising edge of CONVST), the CONVST
signal is brought low, and a conversion is initiated on this falling
edge of the CONVST signal. The conversion takes 2.3 µs and
after this time, the conversion result is latched into the serial
shift register and the part powers down. Therefore, when the
part is operated in Mode 2, the effective conversion time is
equal to the power-up time (1.5 µs) and the SAR conversion
time (2.3 µs).
NOTE: Although the AD7810 takes 1.5 µs to power up after the
rising edge of CONVST, it is not necessary to leave CONVST
high for 1.5 µs after the rising edge before bringing it low to
initiate a conversion. If the CONVST signal goes low before 1.5 µs
in time has elapsed, then the power-up time is timed out inter-
nally and a conversion is then initiated. Hence the AD7810 is
guaranteed to have always powered up before a conversion is
initiated—even if the CONVST pulsewidth is < 1.5 µs. If the
CONVST width is > 1.5 µs, then a conversion is initiated on
the falling edge.
As in the case of Mode 1 operation, the rising edge of the
CONVST pulse enables the serial port of the AD7810 (see
Serial Interface section). If a serial read is initiated soon after
this rising edge (Point “A”), i.e., before the end of the conver-
sion, the result of the previous conversion is shifted out on pin
D
OUT
. In order to read the result of the current conversion, the
user must wait at least 2.3 µs after the falling edge of CONVST
before initiating a serial read. The serial port of the AD7810 is
still functional even though the AD7810 has been powered down.
NOTE: Serial read should not cross the next rising edge of
CONVST.
Because it is possible to do a serial read from the part while it
is powered down, the AD7810 is powered up only to do the
conversion and is immediately powered down at the end of a
conversion. This significantly improves the power consumption
of the part at slower throughput rates—see Power vs. Through-
put Rate section.
SERIAL INTERFACE
The serial interface of the AD7810 consists of three wires, a
serial clock input SCLK, serial port enable CONVST and a
serial data output D
OUT
(see Figure 16). The serial interface
is designed to allow easy interfacing to most microcontrollers,
e.g., PIC16C, PIC17C, QSPI and SPI, without the need for any
gluing logic. When interfacing to the 8051, the SCLK must be
inverted. The Microprocessor Interface section explains how to
interface to some popular microcontrollers.
Figure 16 shows the timing diagram for a serial read from the
AD7810. The serial interface works with both a continuous and
a noncontinuous serial clock. The rising edge of the CONVST
signal resets a counter, which counts the number of serial clocks
to ensure the correct number of bits are shifted out of the serial
shift registers. The SCLK is ignored once the correct number of
bits have been shifted out. In order for another serial transfer to
take place, the counter must be reset by the falling edge of the
10th SCLK. Data is clocked out from the D
OUT
line on the first
rising SCLK edge after the rising edge of the CONVST signal
and on subsequent SCLK rising edges. D
OUT
enters its high
impedance state again on the falling edge of the 10th SCLK.
In multipackage applications, the CONVST signal can be used
as a chip select signal. The serial interface will not shift data out
until it receives a rising edge on the CONVST pin.
CONVST
D
OUT
SCLK
t
8
12 3 45 6 78910
DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0DB9 DB8
t
3
t
4
t
7
t
6
t
5
Figure 16. AD7810 Serial Interface Timing
