AD7823_15 Datasheet, PDF (8/11 Pages) Analog Devices – 2.7 V to 5.5 V, 5s, 8-Bit ADC in 8-Lead microSOIC/DIP

AD7823
POWER-UP TIMES
The AD7823 has a 1.5 µs power-up time. When V_DD is first
connected, the AD7823 is in a low current mode of operation.
In order to carry out a conversion, the AD7823 must first be
powered up. The ADC is powered up by a rising edge on the
CONVST pin. A conversion is initiated on the falling edge of
CONVST. Figure 12 shows how to power up the AD7823 when
V_DD is first connected or after the AD7823 is powered down
using the CONVST pin.
Care must be taken to ensure that the CONVST pin of the
AD7823 is logic low when V_DD is first applied.
V_DD
CONVST
t_POWER-UP
1.5␮s
MODE 1 (CONVST IDLES HIGH)
1µs
V_DD
CONVST
t_POWER-UP
1.5␮s
MODE 2 (CONVST IDLES LOW)
Figure 12. Power-Up Times
POWER VS. THROUGHPUT RATE
By operating the AD7823 in Mode 2, the average power con-
sumption of the AD7823 decreases at lower throughput rates.
Figure 13 shows how the automatic power-down is implemented
using the CONVST signal to achieve the optimum power per-
formance for the AD7823. The AD7823 is operated in Mode 2.
As the throughput rate is reduced, the device remains in its
power-down state for longer, and the average power consump-
tion over time drops accordingly.
CONVST
t_CONVERT
t_POWER-UP ^5␮s
1.5␮s
POWER-DOWN
t _CYCLE
100␮s @ 10kSPS
Figure 13. Automatic Power-Down
For example, if the AD7823 is operated in a continuous sampling
mode with a throughput rate of 10 kSPS, the power consumption
is calculated as follows. The power dissipation during normal
operation is 10.5 mW, V_DD = 3 V. If the power-up time is 1.5 µs
and the conversion time is 5 µs, then the AD7823 can be said to
dissipate 10.5 mW for 6.5 µs (worst case) during each conver-
sion cycle. If the throughput rate is 10 kSPS, the cycle time is
100 µs, and the average power dissipated during each cycle is
(6.5/100) × (10.5 mW) = 683 µW. Figure 2 shows a graph of
Power vs. Throughput.
OPERATING MODES
Mode 1 Operation (High Speed Sampling)
When the AD7823 is used in this mode of operation, the part is
not powered down between conversions. This mode of opera-
tion allows high throughput rates to be achieved. The timing
diagram in Figure 14 shows how this optimum throughput rate
is achieved by bringing the CONVST signal high before the end
of the conversion. It is recommended that the CONVST signal
should go high within 3 µs of conversion starting. This ensures
that the CONVST signal does not go high at the same time the
part is attempting to power down. The AD7823 leaves its tracking
mode and goes into hold on the falling edge of CONVST. A
conversion is also initiated at this time and takes 4 µs typ to
complete. At this point, the result of the current conversion is
latched into the serial shift register, and the state of the CONVST
signal is checked. The CONVST signal should be high at the
end of the conversion to prevent the part from powering down.
t₁
CONVST
t₂ A
B
SCLK
D_OUT
CURRENT CONVERSION
RESULT
Figure 14. Mode 1 Operation Timing
The serial port on the AD7823 is enabled on the rising edge of
the CONVST signal–see Serial Interface section. As explained
earlier, this rising edge should occur before the end of the con-
version process if the part is not to be powered down. A serial
read can take place at any stage after the rising edge of CONVST.
If a serial read is initiated before the end of the current conver-
sion process (i.e., at time “A”), then the result of the previous
conversion is shifted out on the D_OUT pin. It is possible to allow
the serial read to extend beyond the end of a conversion. In this
case, the new data will not be latched into the output shift regis-
ter until the read has finished. If the user waits until the end of
the conversion process, i.e., 4 µs typ after falling edge of CONVST
(Point “B”), before initiating a read, the current conversion
result is shifted out.
–8–
REV. C