ADM1026JST-REEL7 View Datasheet(PDF) - ON Semiconductor

Part Name

Description

Manufacturer

ADM1026JST-REEL7

Complete Thermal System Management Controller

ON Semiconductor 

ADM1026

Voltage Measurement Inputs

The internal structure for all the analog inputs is shown in

Figure 26. Each input circuit consists of an input protection

diode, an attenuator, plus a capacitor to form a first−order

low−pass filter that gives each voltage measurement input

immunity to high frequency noise. The −12 V input also has

a resistor connected to the on−chip reference to offset the

negative voltage range so that it is always positive and can

be handled by the ADC. This allows most popular power

supply voltages to be monitored directly by the ADM1026

without requiring any additional resistor scaling.

AIN0 – AIN5

(0V – 3V)

21.9kΩ

109.4kΩ

4.6pF

AIN6 – AIN9

(0V – 2.5V)

52.5kΩ

4.6pF

+12V

113.5kΩ

21kΩ

9.3pF

–12V

+5V

VREF

17.5kΩ

114.3kΩ

83.5kΩ

50kΩ

9.3pF

MUX

4.6pF

VBAT

49.5kΩ

82.7kΩ

* SEE TEXT

4.5pF

+VCCP

21.9k

109.4kΩ

18.5pF

Figure 26. Voltage Measurement Inputs

Setting Other Input Ranges

AIN0 to AIN9 can easily be scaled to voltages other than

2.5 V or 3.0 V. If the input voltage range is zero to some

positive voltage, all that is required is an input attenuator, as

shown in Figure 27.

R1

AIN(0–9)

VIN

R2

Figure 27. Scaling AIN0 − AIN9

However, when scaling AIN0 to AIN5, it should be noted

that these inputs already have an on−chip attenuator,

because their primary function is to monitor SCSI

termination voltages. This attenuator loads any external

attenuator. The input resistance of the on−chip attenuator

can be between 100 kW and 200 kW. For this tolerance not

to affect the accuracy, the output resistance of the external

attenuator should be very much lower than this, that is, 1 kW

in order to add not more than 1% to the total unadjusted error

(TUE). Alternatively, the input can be buffered using an op

amp.

ǒ Ǔ R1

R2

+

Vfs * 3.0

3.0

ǒfor AIN0 through AIN5Ǔ

(eq. 2)

ǒ Ǔ R1

R2

+

Vfs * 2.5

2.5

ǒfor AIN6 through AIN9Ǔ

(eq. 3)

Negative and bipolar input ranges can be accommodated

by using a positive reference voltage to offset the input

voltage range so that it is always positive. To monitor a

negative input voltage, an attenuator can be used as shown

in Figure 28.

R2

VIN

R1

AIN(0–9)

Figure 28. Scaling and Offsetting AIN0 − AIN9

for Negative Inputs

This offsets the negative voltage so that the ADC always

sees a positive voltage. R1 and R2 are chosen so that the

ADC input voltage is zero when the negative input voltage

is at its maximum (most negative) value, that is:

Ť Ť R1

R2

+

Vfs *

VOS

(eq. 4)

This is a simple and low cost solution, but note the

following:

• Because the input signal is offset but not inverted, the

input range is transposed. An increase in the magnitude

of the negative voltage (going more negative) causes the

input voltage to fall and give a lower output code from

the ADC. Conversely, a decrease in the magnitude of the

negative voltage causes the ADC code to increase. The

maximum negative voltage corresponds to zero output

from the ADC. This means that the upper and lower

limits are transposed.

• For the ADC output to be full scale when the negative

voltage is zero, VOS must be greater than the full−scale

voltage of the ADC, because VOS is attenuated by R1 and

R2. If VOS is equal to or less than the full−scale voltage

of the ADC, the input range is bipolar but not necessarily

symmetrical.

This is a problem only if the ADC output must be full scale

when the negative voltage is zero.

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