DAC128S085EB/NOPB National Semiconductor, DAC128S085EB/NOPB Datasheet - Page 19

• Manufacturer Part Number: DAC128S085EB/NOPB
• Description: BOARD EVALUATION FOR DAC128S085
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of DAC128S085EB/NOPB

• Number Of Dac's: 8
• Number Of Bits: 12
• Outputs And Type: 8, Single Ended
• Data Interface: Serial
• Settling Time: 6µs
• Dac Type: Voltage
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 105°C
• Utilized Ic / Part: DAC128S085
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: DAC128S085EB

2.5.2 ADC Reference
Figure 15 shows Channel A of the DAC128S085 providing the
drive or supply voltage for a bridge sensor. By having the
sensor supply voltage adjustable, the output of the sensor can
be optimized to the input level of the ADC monitoring it. The
output of the sensor is amplified by a fixed gain amplifier stage
with a differential gain of 1 + 2 × (R
this amplifier configuration is the high input impedance seen
by the output of the bridge sensor. The disadvantage is the
poor common-mode rejection ratio (CMRR). The common-
mode voltage (V
A's DAC output. The V
amplifier stage and thus becomes the bias voltage for the in-
put of the ADC121S705. Channel B of the DAC128S085 is
providing the reference voltage to the ADC121S705. The ref-
erence for the ADC121S705 may be set to any voltage from
1V to 5V, providing the widest dynamic range possible.
The reference voltage for Channel A and B is powered by an
external 5V power supply. Since the 5V supply is common to
CM
) of the bridge sensor is half of Channel
CM
is amplified by a gain of 1V/V by the
F
/ R
I
). The advantage of
FIGURE 15. Driving an ADC Reference
FIGURE 14. Industrial Application
19
the sensor supply voltage and the reference voltage of the
ADC, fluctuations in the value of the 5V supply will have a
minimal effect on the digital output code of the ADC. This type
of configuration is often referred to as a "Ratio-metric" design.
For example, an increase of 5% to the 5V supply will cause
the sensor supply voltage to increase by 5%. This causes the
gain or sensitivity of the sensor to increase by 5%. The gain
of the amplifier stage is unaffected by the change in supply
voltage. The ADC121S705 on the other hand, also experi-
ences a 5% increase to its reference voltage. This causes the
size of the ADC's least significant bit (LSB) to increase by 5%.
As a result of the sensor's gain increasing by 5% and the LSB
size of the ADC increasing by the same 5%, there is no net
effect on the circuit's performance. It is assumed that the am-
plifier gain is set low enough to allow for a 5% increase in the
sensor output. Otherwise, the increase in the sensor output
level may cause the output of the amplifiers to clip.
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