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6

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R

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C

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DIFF_MAX

IN_MAX

1

R

V

V

2 R

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DIFF_MAX

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REF

REF

2

3

6

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15

REF3312, REF3318, REF3320, REF3325, REF3330, REF3333

www.ti.com

SBOS392H – AUGUST 2007 – REVISED AUGUST 2019

Product Folder Links: REF3312 REF3318 REF3320 REF3325 REF3330 REF3333

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Copyright © 2007–2019, Texas Instruments Incorporated

Typical Applications (continued)

(1)

Over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted).

When the ratio of R3 and R2 equals the ratio of R7 and R6, Equation 8 simplifies to Equation 10.

That is, if:

(9)

Then:

(10)

Determine the ratio of R1, R2, and R3 by setting A1+ equal to the maximum VDIFF for a full-scale positive or

negative input voltage, VIN_MAX, as shown in Equation 11:

(11)

R2 equals R3; therefore, Equation 11 simplifies to R2 / 2, resulting in Equation 12:

(12)

10.2.1.2.3

Input Filtering

Both inputs feature first-order, low-pass, antialiasing filters that limit the bandwidth and noise of the input signals

applied to the ADC. The A1+ filter is formed by R8 and C1 and the equation for the –3-dB cutoff frequency is

shown in Equation 13:

(13)

The A1– input filter is formed by C2 and the parallel combination of the R6 and R7 resistors, as shown in

Equation 14:

(14)

10.2.1.2.4

Component Selection

10.2.1.2.4.1

Voltage References

The REF33xx series of precision low-power voltage references pair well with the low power consumption of the

MSP430, while achieving the target accuracy goals. The 16-bit converter in the MSP430F2013 accepts an

external reference voltage from 1 V to 1.5 V with a typical reference input of 1.25 V, as shown in Table 2.

Table 2. SD16_A, External Reference Input (MSP430F20x3)(1)

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNIT

VREF(I)

Input voltage range

VCC = 3 V, SD16REFON = 0

1

1.25

1.5

V

IREF(I)

Input current

VCC = 3 V, SD16REFON = 0

50

nA

The REF3312 provides the desired 1.25-V reference voltage for the MSP430 ADC. The accuracy of the

REF3312 output, shown in the Electrical Characteristics, directly affects the accuracy of the entire system and

must be less than the desired unadjusted error goals. The REF3312 maximum ±0.15% initial accuracy

specification is equal to the unadjusted error design goal of 0.15%, indicating that most of the error budget in this

design must be devoted to the reference accuracy.