LM93 datasheet(14/92 Pages) NSC

14 / 92 page

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, VREF 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

The table below summarizes the theoretical voltage values

for value register readings near −12V.

Value Register

∆ from −12V

-13.2068

-10.0563

-13.1821

-9.8505

-13.1574

-9.6448

-13.1327

-9.4390

-13.1080

-9.2332

-13.0833

-9.0275

-13.0586

-8.8217

-13.0339

-8.6159

-13.0092

-8.4101

-12.9845

-8.2044

-12.9598

-7.9986

-12.9351

-7.7928

Value Register

∆ from −12V

-12.9104

-7.5871

-12.8858

-7.3813

-12.8611

-7.1755

-12.8364

-6.9698

-12.8117

-6.7640

-12.7870

-6.5582

-12.7623

-6.3524

-12.7376

-6.1467

-12.7129

-5.9409

-12.6882

-5.7351

-12.6635

-5.5294

-12.6388

-5.3236

-12.6141

-5.1178

-12.5894

-4.9121

-12.5648

-4.7063

-12.5401

-4.5005

-12.5154

-4.2947

-12.4907

-4.0890

-12.4660

-3.8832

-12.4413

-3.6774

-12.4166

-3.4717

-12.3919

-3.2659

-12.3672

-3.0601

-12.3425

-2.8544

-12.3178

-2.6486

-12.2931

-2.4428

-12.2684

-2.2370

-12.2438

-2.0313

-12.2191

-1.8255

-12.1944

-1.6197

-12.1697

-1.4140

-12.1450

-1.2082

-12.1203

-1.0024

-12.0956

-0.7967

-12.0709

-0.5909

-12.0462

-0.3851

-12.0215

-0.1793

-11.9968

0.0264

-11.9721

0.2322

-11.9474

0.4380

-11.9228

0.6437

-11.8981

0.8495

-11.8734

1.0553

-11.8487

1.2610

-11.8240

1.4668

-11.7993

1.6726

-11.7746

1.8784

-11.7499

2.0841

-11.7252

2.2899

-11.7005

2.4957

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