ADM1026JSTZ-R7 datasheet(18/55 Pages) ONSEMI

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ADM1026

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

when the negative voltage is zero.

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.

Figure 29. Scaling and Offsetting AIN0 − AIN9

for Bipolar Inputs

VIN

AIN(0–9)

+VOS

(eq. 5)

R2 +

s *

VOS

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

pin is zero when VIN = negative full scale.

(eq. 6)

R3 +

s *

3.0

for AIN0 through AIN5

R3 +

s *

2.5

for AIN6 through AIN9

(eq. 7)

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 14). 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 = 4.0 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 measurement 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:

(eq. 8)

I +

VBAT

100 kW

TPULSE

TPERIOD

For example, when VBAT = 3.0 V,

(eq. 9)

I +

3.0 V

100 kW

711 ms

273 ms +

78 nA

where TPULSE = VBAT measurement time (711 ms 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 k

W 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.

Figure 30. Equivalent VBAT Input Protection Circuit

ADC

VBAT

DIGITAL

CONTROL

49.5k

82.7k

4.5pF

3k

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 output. 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

is set to 1. This voltage reference output can be used to

provide a stable reference voltage to external circuitry 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 (4m VMAX). Figure 31 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 32, can be used to buffer the

VREF output.

Figure 31. VREF Interface Circuit for VREF Loads < 2 mA

10k

0.1F

VREF

ADM1026

VREF

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