EVAL-ADM1026EB datasheet(18/56 Pages) AD

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ADM1026

Rev. A | Page 18 of 56

Voltage Measurement Inputs

The internal structure for all the analog inputs is shown in

Figure 27. 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.

109.4k

Ω

18.5pF

21.9k

+VCCP

9.3pF

VREF

17.5k

Ω

114.3k

Ω

–12V

49.5k

Ω

82.7k

Ω

4.5pF

VBAT

* SEE TEXT

AIN0 – AIN5

(0V – 3V)

109.4k

Ω

4.6pF

21.9k

Ω

AIN6 – AIN9

(0V – 2.5V)

4.6pF

52.5k

Ω

50k

Ω

4.6pF

83.5k

Ω

+5V

21k

Ω

9.3pF

113.5k

Ω

+12V

MUX

Figure 27. Voltage Measurement Inputs

Setting Other Input Ranges

AIN0 to AIN9 can easily be scaled to voltages other than 2.5 V or

3 V. If the input voltage range is zero to some positive voltage, all

that is required is an input attenuator, as shown in Figure 28.

VIN

AIN(0–9)

Figure 28. 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 kΩ and 200 kΩ. 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 kΩ in order to add not more than 1% to the

total unadjusted error (TUE). Alternatively, the input can be

buffered using an op amp.

(

)()

IN5

IN0

for

−

(

)()

IN9

IN6

for

−

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 29.

VIN

AIN(0–9)

Figure 29. 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:

−

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.

EVAL-ADM1026EB Datasheet(PDF) 18 Page - Analog Devices

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