Datasheet
_3dB A1
6
2
1
f
R
2 C
2
S
§ ·
¨ ¸
© ¹
3dB_A1+
8 1
1
f
2 R C
S
2
DIFF_MAX IN_MAX
1
R
V V
2 R
§ ·
¨ ¸
© ¹
||
2 3
DIFF_MAX IN_MAX
1
R R
A1+ V V
R
§ ·
¨ ¸
© ¹
||
2 3
DIFF IN
1
R R
V V
R
§ ·
¨ ¸
© ¹
3
7
REF REF REF
2 3 6 7
R
R
1
V V V
R R R R 2
§ · § ·
¨ ¸ ¨ ¸
© ¹ © ¹
REF3312
,
REF3318
,
REF3320
,
REF3325
,
REF3330
,
REF3333
SBOS392D –AUGUST 2007–REVISED JUNE 2014
www.ti.com
Typical Applications (continued)
When the ratio of R
3
and R
2
equals the ratio of R
7
and R
6
, Equation 8 simplifies to Equation 10.
That is, if:
(9)
Then:
(10)
Determine the ratio of R
1
, R
2
, and R
3
by setting A1+ equal to the maximum V
DIFF
for a full-scale positive or
negative input voltage, VIN_MAX, as shown in Equation 11:
(11)
R
2
equals R
3
; therefore, Equation 11 simplifies to R
2
/ 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 R
8
and C
1
and the equation for the –3-dB cutoff frequency is
shown in Equation 13:
(13)
The A1– input filter is formed by C
2
and the parallel combination of the R
6
and R
7
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 1.
Table 1. SD16_A, External Reference Input (MSP430F20x3)
(1)
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
V
REF(I)
Input voltage range VCC = 3 V, SD16REFON = 0 1 1.25 1.5 V
I
REF(I)
Input current VCC = 3 V, SD16REFON = 0 50 nA
(1) Over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted).
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.
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Product Folder Links: REF3312 REF3318 REF3320 REF3325 REF3330 REF3333