Datasheet

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www.ti.com
ADS7826
ADS7827
ADS7829
SLAS388–JUNE 2003
There is an important distinction between the power The reference should be similarly bypassed with a
down mode that is entered after a conversion is 0.1-µF capacitor. Again, a series resistor and large
complete and the full power-down mode which is capacitor can be used to lowpass filter the reference
enabled when CS is HIGH. While both shutdown the voltage. If the reference voltage originates from an
analog section, the digital section is completely op-amp, be careful that the op-amp can drive the
shutdown only when CS is HIGH. Thus, if CS is left bypass capacitor without oscillation (the series
LOW at the end of a conversion and the converter is resistor can help in this case). Keep in mind that
continually clocked, the power consumption is not as while the ADS7826/27/29 family draws very little
low as when CS is HIGH. current from the reference on average, there are still
instantaneous current demands placed on the
Power dissipation can also be reduced by lowering
external reference circuitry.
the power supply voltage and the reference voltage.
The ADS7826/27/29 family operates over a V
CC
Also, keep in mind that the ADS7826/27/29 family
range of 2.0 V to 5.25 V. However, at voltages below offers no inherent rejection of noise or voltage
2.7 V, the converter does not run at the maximum variation in regards to the reference input. This is of
sample rate. See the typical performance curves for particular concern when the reference input is tied to
more information regarding power supply voltage and the power supply. Any noise and ripple from the
maximum sample rate. supply appears directly in the digital results. While
high frequency noise can be filtered out as described
LAYOUT
in the previous paragraph, voltage variation due to
the line frequency (50 Hz or 60 Hz), can be difficult to
For optimum performance, care should be taken with
remove.
the physical layout of the ADS7826/27/29 family
circuitry. This is particularly true if the reference
The GND pin on the ADS7826/27/29 family must be
voltage is low and/or the conversion rate is high. At a
placed on a clean ground point. In many cases, this
125-kHz to 250-kHz conversion rate, the
is the analog ground. Avoid connecting the GND pin
ADS7826/27/29 family makes a bit decision every
too close to the grounding point for a microprocessor,
800 ns to 400 ns. That is, for each subsequent bit
microcontroller, or digital signal processor. If needed,
decision, the digital output must be updated with the
run a ground trace directly from the converter to the
results of the last bit decision, the capacitor array
power supply connection point. The ideal layout
appropriately switched and charged, and the input to
includes an analog ground plane for the converter
the comparator settled, for example the ADS7829, to
and associated analog circuitry.
a 12-bit level all within one clock cycle.
APPLICATION CIRCUITS
The basic SAR architecture is sensitive to spikes on
the power supply, reference, and ground connections Figure 39 and Figure 40 show some typical
that occur just prior to latching the comparator output. application circuits the ADS7826/27/29 family. Figure
Thus, during any single conversion for an n-bit SAR 39 uses an ADS7826/27/29 and a multiplexer to
converter, there are n windows in which large provide for a flexible data acquisition circuit. A
external transient voltages can easily affect the resistor string provides for various voltages at the
conversion result. Such spikes might originate from multiplexer input. The selected voltage is buffered
switching power supplies, digital logic, and high and driven into V
ref
. As shown in Figure 39, the input
power devices, to name a few. This particular source range of the ADS7826/27/29 family programmable to
of error can be very difficult to track down if the glitch 100 mV, 200 mV, 300 mV, or 400 mV. The 100-mV
is almost synchronous to the converter’s DCLOCK range would be useful for sensors such as
signal—as the phase difference between the two thermocouple shown.
changes with time and temperature, causing sporadic
Figure 39 shows a basic data acquisition system. The
misoperation.
ADS7826/27/29 family input range is 0 V to V
CC
, as
With this in mind, power to the ADS7826/27/29 family the reference input is connected directly to the power
should be clean and well bypassed. A 0.1-µF ceramic supply. The 5-Ω resistor and 1-µF to 10-µF capacitor
bypass capacitor should be placed as close to the filters the microcontroller noise on the supply, as well
ADS7826/27/29 family package as possible. In as any high-frequency noise from the supply itself.
addition, a 1-µ to 10-µF capacitor and a 5-Ω or 10-Ω The exact values should be picked such that the filter
series resistor may be used to lowpass filter a noisy provides adequate rejection of the noise.
supply.
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