Rainbow Electronics LM93 - Manual (Page 14)

12.0 Functional Description
(Continued)
Required External Level Shifting
Resistors for −12V Power Input
20068210
The +3.3V standby voltage is used as a reference for the
level shifting. Therefore, the tolerance of this voltage directly
effects the accuracy of the −12V reading. To minimize ratio
errors, a tolerance of better than
± 1% should be used. It is
recommended that the equivalent thevenin resistance of the
divider is between 1k and 7k to minimize errors caused by
leakage currents at extreme temperatures. To calculate the
ratio of R1 to R2 use this equation:
where V
IN
is the nominal input voltage of −12V, V
REF
is the
reference voltage of +3.3V and AD_IN is the voltage required
at the AD input for a
1 ⁄
4
scale reading or 0.309V.
Therefore, for this case:
Using standard 1% resistor values for R1 of 5.76 k Ω and R2
of 1.4 k Ω yields an R1 to R2 ratio of 4.1143.
The input voltage V
IN
can be calculated using the value
register reading (VR) using this equation:
The table below summarizes the theoretical voltage values
for value register readings near −12V.
Value Register V
IN
% ∆ from −12V
15 -13.2068 -10.0563
16 -13.1821 -9.8505
17 -13.1574 -9.6448
18 -13.1327 -9.4390
19 -13.1080 -9.2332
20 -13.0833 -9.0275
21 -13.0586 -8.8217
22 -13.0339 -8.6159
23 -13.0092 -8.4101
24 -12.9845 -8.2044
25 -12.9598 -7.9986
26 -12.9351 -7.7928
Value Register V
IN
% ∆ from −12V
27 -12.9104 -7.5871
28 -12.8858 -7.3813
29 -12.8611 -7.1755
30 -12.8364 -6.9698
31 -12.8117 -6.7640
32 -12.7870 -6.5582
33 -12.7623 -6.3524
34 -12.7376 -6.1467
35 -12.7129 -5.9409
36 -12.6882 -5.7351
37 -12.6635 -5.5294
38 -12.6388 -5.3236
39 -12.6141 -5.1178
40 -12.5894 -4.9121
41 -12.5648 -4.7063
42 -12.5401 -4.5005
43 -12.5154 -4.2947
44 -12.4907 -4.0890
45 -12.4660 -3.8832
46 -12.4413 -3.6774
47 -12.4166 -3.4717
48 -12.3919 -3.2659
49 -12.3672 -3.0601
50 -12.3425 -2.8544
51 -12.3178 -2.6486
52 -12.2931 -2.4428
53 -12.2684 -2.2370
54 -12.2438 -2.0313
55 -12.2191 -1.8255
56 -12.1944 -1.6197
57 -12.1697 -1.4140
58 -12.1450 -1.2082
59 -12.1203 -1.0024
60 -12.0956 -0.7967
61 -12.0709 -0.5909
62 -12.0462 -0.3851
63 -12.0215 -0.1793
64 -11.9968 0.0264
65 -11.9721 0.2322
66 -11.9474 0.4380
67 -11.9228 0.6437
68 -11.8981 0.8495
69 -11.8734 1.0553
70 -11.8487 1.2610
71 -11.8240 1.4668
72 -11.7993 1.6726
73 -11.7746 1.8784
74 -11.7499 2.0841
75 -11.7252 2.2899
76 -11.7005 2.4957
LM93
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