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| REF3312 Datasheet, PDF (13/32 Pages) Texas Instruments – 3.9μA, SC70-3, 30ppm/°C Drift VOLTAGE REFERENCE | |||
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REF3312, REF3318, REF3320, REF3325, REF3330, REF3333
SBOS392E â AUGUST 2007 â REVISED OCTOBER 2014
Typical Applications (continued)
10.2.1.1 Design Requirements
The design requirements are as follows:
⢠Supply voltage: 3.3 V
⢠Maximum input voltage: ±6 V
⢠Specified input voltage: ±5 V
⢠ADC reference voltage: 1.25 V
10.2.1.2 Detailed Design Procedure
Figure 19 depicts a simplified schematic for this design showing the MSP430 ADC inputs and full input
conditioning circuitry. The ADC is configured for a bipolar measurement where final conversion result is the
differential voltage, VDIFF, between the positive and negative ADC inputs, A1+ and A1â. The bipolar, ground-
referenced input signal must be level-shifted and attenuated by the op amp so that the output is biased to VREF /
2 and has a differential voltage that is within the ±VREF / 2 input range of the ADC. The transfer function for the
op-amp circuit simplifies to Equation 4.
A1
§
¨
©
R3
R2 R3
·
¸
¹
VREF
§
¨
R2
||
R3
© R1
·
¸
¹
VIN
where
⢠R1 = R4
⢠R5 = R2 || R3
(4)
The voltage applied to the negative ADC input, A1â, is based on the resistor divider formed by R6 and R7 and is
set to VREF / 2 by setting R6 equal to R7, as shown in Equation 5.
A1
§
¨
©
R7
R6 R7
·
¸
¹
VREF
VREF
2
(5)
10.2.1.2.1 Op Amp Level-Shift Design
The ratio of R2, R3, and the VREF voltage determines the voltage on the output of the op amp when the
differential input is 0 V. Select the components so that VOUT is equal to the VREF / 2 voltage when VIN is equal to
0 V, as shown in Equation 6.
A1
VREF
2
§
¨
©
R3
R2 R3
·
¸
¹
VREF
where
⢠VIN = 0 V
⢠R2 = R3
(6)
Solve for the value of R5 by setting R3 equal to R2 in Equation 4, as shown in Equation 7:
R5
§
¨
©
R2
R2
 R2
R2
·
¸
¹
R22
2 Â R2
R2
2
(7)
10.2.1.2.2 Differential Input Attenuator Design
VDIFF is the difference between the two inputs, as shown in Equation 8:
VDIFF
A1+ A1
§
¨
©
R3
R2 R3
·
¸
¹
VREF
§
¨
R2
||
R3
© R1
·
¸
¹
VIN
VREF
2
(8)
Copyright © 2007â2014, Texas Instruments Incorporated
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Product Folder Links: REF3312 REF3318 REF3320 REF3325 REF3330 REF3333
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