LM93 datasheet(12/95 Pages) TI1

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(-12 - 3.3)

(0.309 - 3.3)

- 1 = 4.11535

(VIN - VREF)

(AD_IN - VREF)

- 1

VIN

AD_IN15

3.3V

±1%

0.927

- 1 = 11.04498

VIN

to AD_IN1-

LM93

SNAS210E – DECEMBER 2003 – REVISED MARCH 2013

www.ti.com

3.5

RECOMMENDED EXTERNAL SCALING RESISTORS FOR +12V POWER RAILS

The +12V inputs require external scaling resistors. The resistors need to scale 12V down to 0.927V.

Figure 3-1. Required External Scaling

Resistors for +12V Power Input

To calculate the required ratio of R1 to R2 use this equation:

(1)

It is recommended that the equivalent thevenin resistance of the divider be between 1k and 7k to minimize

errors caused by leakage currents at extreme temperatures. The best values for the resistors are:

R1=13.7 k

Ω and R2=1.15 kΩ. This yields a ratio of 11.94498, which has a +0.27% deviation from the

theoretical. It is also recommended that the resistors have ±1% tolerance or better.

Each LSB in the voltage value registers has a weight of 12V / 192 = 62.5 mV. To calculate the actual

voltage of the +12V power input, use the following equation:

VIN = (8-bit value register code) x (62.5 mV)

(2)

3.6

RECOMMENDED EXTERNAL SCALING CIRCUIT FOR

−12V POWER INPUT

The

−12V input requires external resistors to level shift the nominal input voltage of −12V to +0.309V.

Figure 3-2. Required External Level Shifting

Resistors for

−12V Power Input

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:

(3)

where VIN 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 ¼ scale reading or 0.309V.

Therefore, for this case:

(4)

Using standard 1% resistor values for R1 of 5.76 k

Ω and R2 of 1.4 kΩ yields an R1 to R2 ratio of 4.1143.

The input voltage VIN can be calculated using the value register reading (VR) using this equation:

Functional Description

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