Datasheet

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SBAS253E − MAY 2003 − REVISED JULY 2006
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22
2.5 8-Bit Digital-to-Analog Converter
A voltage between 0.5V to 2.5V is required at DAIN (pin 30) to set the range of the window comparators; this
can be accomplished with the 8-bit DAC. The DAC value is programmed via the digital interface. Input code
00
H
corresponds to a DAC output voltage of 0V. The full-scale value (FF
H
) is at 2.49V (internal reference minus
1LSB).
The impedance of the output is typically 10kΩ; the output impedance is independent of the output voltage.
The DAC output is connected to DAOUT (pin 29). The settling time (t
s
) is dependent on the external
capacitance (C
e
) on this pin, and can be calculated to:
t
s
+ 10kW @ C
e
@ (n ) 1) @ ln(2)
In this equation, n is equal to 8, the resolution of the DAC. The output impedance also limits the output current.
This current should not exceed 0.5µA (0.5µA
S10kΩ = 5mV).
DAOUT and DAIN can be shorted. A capacitor (typically 0.1µF) can be used to low-pass the DAC output;
however, this low-pass configuration is not required.
2.6 Internal Reference
The internal reference, REFOUT (pin 96), provides the 2.5V required for the reference input of the ADCs at
REFIN (pin 97). An internal buffer with a high impedance output drives the reference output pin. This internal
buffer is optimized to reject glitches at the reference pin. Any capacitor can be connected to the REFOUT pin
in able to reduce noise. It is recommended that a 0.1µF capacitor be connected between the REFOUT (pin
96) and the SGND (pin 13). The Signal ground, SGND, is used internally as a negative reference. The
reference voltage is considered a differential voltage between this ground and REFOUT.
Normally, the REFOUT and REFIN pins are both shorted. The internal reference provides an excellent
temperature drift, typically 20ppm, and an initial accuracy of 2.5V±20mV at 25°C. If this does not provide the
required accuracy for an application, then an external reference can be connected to the REFIN pin.
2.7 Grounding
Optimal test results were achieved with a solid ground plane: linearity, offset, and noise performance each
showed improvement. During PCB layout, care should be taken that the return currents do not cross any
sensitive areas or signals.
Digital signals that interface with the ADS7869 are referenced to the solid ground plane. ESD protection
diodes, inside the ADS7869, start conducting if the grounds are separated and the digital inputs go below
–0.3V; this includes short glitches. Current will flow through the substrate of the ADS7869 and will disturb the
analog performance.
2.8 Supply
The ADS7869 has two separate supplies, BV
DD
(pins 48 and 78) and AV
DD
(pins 8, 18, 28, 85 and 98).
BV
DD
is used as a digital pad supply only, and is in the range of 2.7V to 5.5V. This allows the ADS7869 to
interface with all state-of-the-art processors and controllers. BV
DD
should be filtered in order to limit the noise
energy from the external digital circuitry to the ADS7869. The current through BV
DD
is far below 5mA;
depending on the external load, a 10Ω to 100Ω resistor can be placed between the external digital circuitry
and the ADS7869. Bypass capacitors (two 0.1µF and one 10µF) should be placed between the two BV
DD
pins
and the ground plane.
AV
DD
supplies the internal circuitry, and can vary from 4.5 to 5.5V. It is not possible to use a passive filter
between the digital board supply of the application and the AV
DD
pins, because the supply current of the
ADS7869 is typically 45mA. In order to generate the analog supply voltage for the ADS7869 and the
necessary analog front-end, a linear regulator (7805 family) is recommended. Bypass capacitors of 0.1µF
should be placed between all AV
DD
pins and the ground plane. Bypass capacitors of 10µF should be placed
between two AV
DD
pins and the ground plane.
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