ADBMS2950BCCSZ datasheet(83/97 Pages) AD

83 / 97 page

Data Sheet

ADBMS2950B

Rev. 0 | Page 83 of 97

stays above BAT− all the time, thus it is not required to bias the

connected divider to VREF1P25, instead, it can also be

connected to GND (BAT−) directly according to Figure 65.

For Figure 46, the differential LINK voltage (LINK+ vs. LINK−)

is measured and calculated as following:

VLINK+ = VV2,VREF1P25 ÷ gLP + VREF1P25

VLINK- = VV4,VREF1P25 ÷ gLN + VREF1P25

VLINK = VLINK+ − VLINK− = VV2,VREF1P25 ÷ gLP − VV4,VREF1P25 ÷ gLN

where gLP is the gain factor of the LINK+ divider and gLN the

gain factor of the LINK− divider.

If both dividers have the same ratio, gLP = gLN = g the equation

can be simplified:

VLINK = (VV2,VREF1P25 − VV4,VREF1P25) ÷ g = VV2,V4 ÷ g

Which allows a single differential measurement of VV2,V4, V2 vs.

V4 (VS2 set to 0b11) that yields the differential LINK voltage.

Input Leakage Current

All voltage inputs of the ADBMS2950B are buffered but still

exhibit some leakage current. The input leakage current ILEAK

during measurements of any pin is listed under the Input

Current entry in Table 3 and Table 5. The source impedance

RSRC of the resistive divider multiplied by ILEAK defines the

additional absolute voltage error at the ADC input. With a

source impedance of 10 kΩ this error is in the range of 150 μV

and translates with the inverse of the resistor-divider gain factor

g to a measurement error on the high side. At an ADC input

signal of 1.0V, it translates to a gain error of less than 0.02% and

can usually be neglected.

The input leakage current ILEAK changes with input signals and

over temperature but stays within the limits listed in Table 3

and Table 5. It can be calibrated only partially during a post-

production board test procedure. The worst-case error is

calculated as follows.

Source impedance:

RSRC = Rlow × Rhigh ÷ (Rlow + Rhigh)

Absolute measurement error at ADC input due to pin leakage

current:

eabs,ADC = RSRC × ILEAK

Absolute measurement error at high-voltage input due to pin

leakage current:

eabs,HV = eabs,ADC ÷ g

Note that the pin V4 has a higher maximum input-leakage

current specification.

VBATx Connection Options

Figure 67, Figure 68, Figure 69, and Figure 70 show all possible

configuration options of the VBATx input connections.

Figure 67. VBATx Configuration for Unipolar Differential Measurement

Figure 68. VBATx Configuration for Unipolar Single-Ended Redundant

Measurement

Figure 69. VBATx Configuration for Bipolar Differential Measurement

Figure 70 is shown for completeness on the VBATx connection

options. It is possible from a functional perspective with the

downside that single-ended measurements (here, VBATx vs.

GND) at inputs connected to dividers that are biased to

VREF1P25 suffer from measurement errors in noisy

environments.

Any measurement error of VREF1P25 (for example, deviation

in voltage between the time of the Vx and the VREF1P25

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