EVAL-ADM1026EB datasheet(19/56 Pages) AD

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ADM1026

Rev. A | Page 19 of 56

Symmetrical bipolar input ranges can be accommodated easily

by making VOS equal to the full-scale voltage of the analog input,

and by adding a third resistor to set the positive full scale.

VIN

AIN(0–9)

+VOS

Figure 30. Scaling and Offsetting AIN0 − AIN9 for Bipolar Inputs

−

Note that R3 has no effect as the input voltage at the device pin

is zero when VIN = negative full scale.

(

)()

IN5

IN0

for

−

(

)()

IN9

IN6

for

−

Also, note that R2 has no effect as the input voltage at the device

pin is equal to VOS when VIN = positive full scale.

Battery Measurement Input (VBAT)

The VBAT input allows the condition of a CMOS backup battery

to be monitored. This is typically a lithium coin cell such as a

CR2032. The VBAT input is accurate only for voltages greater

than 1.5 V (see Figure 15). Typically, the battery in a system is

required to keep some device powered on when the system is in

a powered-off state. The VBAT measurement input is specially

designed to minimize battery drain. To reduce current drain

from the battery, the lower resistor of the VBAT attenuator is not

connected, except whenever a VBAT measurement is being made.

The total current drain on the VBAT pin is 80 nA typical (for a

maximum VBAT voltage = 4 V), so a CR2032 CMOS battery

functions in a system in excess of the expected 10 years. Note

that when a VBAT measurement is not being made, the current

drain is reduced to 6 nA typical. Under normal voltage meas-

urement operating conditions, all measurements are made in a

round-robin format, and each reading is actually the result of

16 digitally averaged measurements. However, averaging is not

carried out on the VBAT measurement to reduce measurement

time and therefore reduce the current drain from the battery.

The VBAT current drain when a measurement is being made is

calculated by

PERIOD

PULSE

BAT

100

For example, when VBAT = 3 V,

273

711

100

where TPULSE = VBAT measurement time (711 µs typical),

TPERIOD = time to measure all analog inputs (273 ms typical),

and VBAT input battery protection.

VBAT Input Battery Protection

In addition to minimizing battery current drain, the VBAT

measurement circuitry was specifically designed with battery

protection in mind. Internal circuitry prevents the battery from

being back-biased by the ADM1026 supply or through any

other path under normal operating conditions. In the unlikely

event of a catastrophic ADM1026 failure, the ADM1026

includes a second level of battery protection including a series

3 kΩ resistor to limit current to the battery, as recommended by

UL. Thus, it is not necessary to add a series resistor between the

battery and the VBAT input; the battery can be connected directly

to the VBAT input to improve voltage measurement accuracy.

ADC

VBAT

DIGITAL

CONTROL

49.5k

Ω

82.7k

Ω

4.5pF

Ω

Figure 31. Equivalent VBAT Input Protection Circuit

Reference Output (VREF)

The ADM1026 offers an on-chip reference voltage (Pin 24) that

can be used to provide a 1.82 V or 2.5 V reference voltage out-

put. This output is buffered and specified to sink or source a

load current of 2 mA. The reference voltage outputs 1.82 V if

Bit 2 of Configuration Register 3 (Address 07h) is 0; it outputs

2.5 V when this bit is set to 1. This voltage reference output can

be used to provide a stable reference voltage to external cir-

cuitry such as LDOs. The load regulation of the VREF output is

typically 0.15% for a sink current of 2 mA and 0.15% for 2 mA

source current. There may be some ripple present on the VREF

output that requires filtering (±4 m VMAX). Figure 32 shows the

recommended circuitry for the VREF output for loads less than

2 mA. For loads in excess of 2 mA, external circuitry, such as

that shown in Figure 33, can be used to buffer the VREF output.

10k

Ω

0.1

µF

VREF

ADM1026

VREF

Figure 32. VREF Interface Circuit for VREF Loads < 2 mA

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