Datasheet

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LAYOUT
APPLICATION CIRCUITS
ADS7822
SBAS062C – JANUARY 1996 – REVISED AUGUST 2007
Also, keep in mind that the ADS7822 offers no
inherent rejection of noise or voltage variation in
For optimum performance, care should be taken with
regards to the reference input. This is of particular
the physical layout of the ADS7822 circuitry. This is
concern when the reference input is tied to the power
particularly true if the reference voltage is low and/or
supply. Any noise and ripple from the supply will
the conversion rate is high. At a 75kHz conversion
appear directly in the digital results. While
rate, the ADS7822 makes a bit decision every 830ns.
high-frequency noise can be filtered out as described
If the supply range is limited to 4.75V to 5.25V, then
in the previous paragraph, voltage variation due to
up to a 200kHz conversion rate can be used, which
the line frequency (50Hz or 60Hz), can be difficult to
reduces the bit decision time to 312ns. That is, for
remove.
each subsequent bit decision, the digital output must
be updated with the results of the last bit decision, The GND pin on the ADS7822 should be placed on a
the capacitor array appropriately switched and clean ground point. In many cases, this will be the
charged, and the input to the comparator settled to a analog ground. Avoid connecting the GND pin too
12-bit level all within one clock cycle. close to the grounding point for a microprocessor,
microcontroller, or digital signal processor. If needed,
The basic SAR architecture is sensitive to spikes on
run a ground trace directly from the converter to the
the power supply, reference, and ground connections
power-supply connection point. The ideal layout will
that occur just prior to latching the comparator output.
include an analog ground plane for the converter and
Thus, during any single conversion for an n-bit SAR
associated analog circuitry.
converter, there are n windows in which large
external transient voltages can easily affect the
conversion result. Such spikes might originate from
switching power supplies, digital logic, and
Figure 27 and Figure 28 show some typical
high-power devices, to name a few. This particular
application circuits for the ADS7822. Figure 27 uses
source of error can be very difficult to track down if
an ADS7822 and a multiplexer to provide for a
the glitch is almost synchronous to the converter
flexible data acquisition circuit. A resistor string
DCLOCK signal because the phase difference
provides for various voltages at the multiplexer input.
between the two changes with time and temperature,
The selected voltage is buffered and driven into V
REF
.
causing sporadic misoperation.
As shown in Figure 27 , the input range of the
ADS7822 is programmable to 100mV, 200mV,
With this in mind, power to the ADS7822 should be
300mV, or 400mV. The 100mV range would be
clean and well-bypassed. A 0.1 μ F ceramic bypass
useful for sensors such as the thermocouple shown.
capacitor should be placed as close to the ADS7822
package as possible. In addition, a 1 μ F to 10 μ F
Figure 28 shows a basic data acquisition system. The
capacitor and a 5 Ω or 10 Ω series resistor can be
ADS7822 input range is 0V to V
CC
, as the reference
used to lowpass filter a noisy supply.
input is connected directly to the power supply. The
5 Ω resistor and 1 μ F to 10 μ F capacitor filter the
The reference should be similarly bypassed with a
microcontroller noise on the supply, as well as any
0.1 μ F capacitor. Again, a series resistor and large
high-frequency noise from the supply itself. The exact
capacitor can be used to lowpass filter the reference
values should be picked such that the filter provides
voltage. If the reference voltage originates from an op
adequate rejection of the noise.
amp, be careful that the op amp 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.
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Product Folder Link(s): ADS7822