ADS7822EB datasheet(13/20 Pages) TI

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ADS7822

SBAS062A

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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.

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 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.

With this in mind, power to the ADS7822 should be clean

and well-bypassed. A 0.1

µF ceramic bypass capacitor should

be placed as close to the ADS7822 package as possible. In

addition, a 1 to 10

µF capacitor and a 5Ω or 10Ω series

resistor may be used to lowpass filter a noisy supply.

The reference should be similarly bypassed with a 0.1

µF

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

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.

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.

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.

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 5

Ω resistor and 1µF to

µF 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.

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