This technique can be used to lower the power dissipation (or
to increase the conversion rate) in those applications where
an analog signal is being monitored until some condition
becomes true. For example, if the signal is outside a prede-
termined range, the full 12-bit conversion result may not be
needed. If so, the conversion can be terminated after the first
n-bits, where n might be as low as 3 or 4. This results in lower
power dissipation in both the converter and the rest of the
system, as they spend more time in the power-down mode.
can drive the bypass capacitor without oscillation (the series
resistor can help in this case). Keep in mind that while the
ADS7822 draws very little current from the reference on
average, there are still instantaneous current demands placed
on the external reference circuitry.
Also, keep in mind that the ADS7822 offers no inherent
rejection of noise or voltage variation in regards to the
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital
results. While high frequency noise can be filtered out as
described in the previous paragraph, voltage variation due to
the line frequency (50Hz or 60Hz), can be difficult to
remove.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7822 circuitry. This will be
particularly true if the reference voltage is low and/or the
conversion rate is high. At a 75kHz conversion rate, the
ADS7822 makes a bit decision every 830ns. That is, for each
subsequent bit decision, the digital output must be updated
with the results of the last bit decision, the capacitor array
appropriately switched and charged, and the input to the
comparator settled to a 12-bit level all within one clock cycle.
The GND pin on the ADS7822 should be placed on a clean
ground point. In many cases, this will be the “analog”
ground. Avoid connecting the GND pin too close to the
grounding point for a microprocessor, microcontroller, or
digital signal processor. If needed, run a ground trace directly
from the converter to the power supply connection point. The
ideal layout will include an analog ground plane for the
converter and associated analog circuitry.
The basic SAR architecture is sensitive to spikes on the
power supply, reference, and ground connections that occur
just prior to latching the comparator output. Thus, during any
single conversion for an n-bit SAR converter, there are n
“windows” in which large external transient voltages can
easily affect the conversion result. Such spikes might origi-
nate from switching power supplies, digital logic, and high
power devices, to name a few. This particular source of error
can be very difficult to track down if the glitch is almost
synchronous to the converter’s DCLOCK signal—as the
phase difference between the two changes with time and
temperature, causing sporadic misoperation.
APPLICATION CIRCUITS
Figures 6 and 7 show some typical application circuits for
the ADS7822. Figure 6 uses an ADS7822 and a multiplexer
to provide for a flexible data acquisition circuit. A resistor
string provides for various voltages at the multiplexer input.
The selected voltage is buffered and driven into VREF. As
shown in Figure 6, the input range of the ADS7822 is
programmable to 100mV, 200mV, 300mV, or 400mV. The
100mV range would be useful for sensors such as the
thermocouple shown.
With this in mind, power to the ADS7822 should be clean
and well-bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the ADS7822 package as possible. In
addition, a 1 to 10µF capacitor and a 5Ω or 10Ω series
resistor may be used to lowpass filter a noisy supply.
Figure 7 shows a basic data acquisition system. The ADS7822
input range is 0V to VCC, as the reference input is connected
directly to the power supply. The 5Ω resistor and 1µF to
10µF capacitor filter the microcontroller “noise” on the
supply, as well as any high-frequency noise from the supply
itself. The exact values should be picked such that the filter
provides adequate rejection of the noise.
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to lowpass filter the reference voltage. If the reference
voltage originates from an op amp, be careful that the op amp
ADS7822
SBAS062A
13
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