ADBMS2950BCCSZ datasheet(41/97 Pages) AD

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Data Sheet

ADBMS2950B

Rev. 0 | Page 41 of 97

zero, the first measurement is delayed by tSOAK after the ADV

command. If open-wire detection is enabled by setting the OW

bits, the current sources are switched on immediately after the

multiplexer is set to the selected input and stay enabled for the

SOAK and the ADC conversion time. If the VCH parameter is

set to any value greater than eight, the VxADCs measure

multiple channels, one after the other.

Table 58. V1ADC and V2ADC Multichannel Options

VCH

Sequence

Time

VCH0-VCH8

2.39 ms

VCH1, VCH3, VCH5

0.80 ms

VCH0-VCH5

1.59 ms

VCH0-VCH3

1.06 ms

VCH0, VCH2, VCH4

0.80 ms

VCH4-VCH7

1.06 ms

VCH5-VCH7

0.80 ms

The SOAK time is applied to every individual measurement.

Consequently, multichannel conversions take n times as long as

single conversions, where n is the number of specified channels.

The following formula can be used to calculate the total end of

conversion (EOC) depending on the number of channels (n)

and tSOAK (see SOAK configuration in Table 70):

tEOC = n × (549 ÷ OSC1 + tSOAK)

With the biggest SOAK time configured, the typical overall

measurement time is 9 × (0.265 ms + 150 ms) = 1.35 seconds.

VxADC Open-Wire Current Sources

As shown in the block diagram, the ADBMS2950B features 10

μA pull-up current sources at the inputs of the voltage

measurement paths. Activating these current sources allows the

detection of a broken input connection or multiplexer. The

open-wire current sources are controlled by the OW bits. For

more details, see Table 50.

As the open-wire current sources are connected after internal

resistors of 2.3 kΩ, which activate them changes the respective

ADC reading by about 23 mV, either in the positive (OW = 01)

or negative (OW = 10) direction.

The open-wire current sources of the V2ADC are only active

when that ADC is measuring V9 or V10. As a result, when

measuring V1 to V6, the V2ADC results are not affected by the

internal voltage drop of 23 mV, while they are affected for the

measurements of V9 and V10. In addition to internal voltage

drop, all measurements on Vx pins are altered by the voltage

drop of the pull-up current across the external source

impedance of the respective pin to GND. The SOAK time

configuration allows adapting to the settling time of the

external RC filters by activating the current for the configured

time duration before the ADC conversion, the current source is

active during the conversion itself and during the SOAK time

leading up to it.

The open-wire current sources of V1ADC and V2ADC are not

active when measuring VREF2.

VBxADC and VxADC Transfer Function

The transfer functions of the V1ADC and VB1ADC are

different from those of the V2ADC and VB2ADC. This has an

impact on the digital output codes that can be reached.

All VxADCs and VBxADCs calculate the ratio of their input

signal to VREF1. These ratios cannot exceed the −1 to +1

ranges.

The V1ADC and VB1ADC have an LSB size of +100 µV. At the

nominal VREF1 voltage of 3.2V, the highest digital output code

that can be reached is 3.2V ÷ 100 µV = 32000, which is lower

than the highest code that can be represented in the register

(+32767).

A similar situation exists for negative inputs. Depending on the

value of VREF1, these ADCs have a range of digital output

codes that cannot be reached. The V2ADC and VB2ADCs have

an LSB size of −85 µV. The largest positive input voltage that

can be digitally represented is -32768 × −85 µV = 2.785V, which

is lower than the guaranteed minimum value of VREF1.

Therefore, these ADCs do not have any unreachable codes.

Figure 30 and Figure 31 visualize those details in the ADC

transfer functions. V1ADC is equivalent to VB1ADC, and

V2ADC is equivalent to VB2ADC.

Having a set of unreachable codes can be helpful to distinguish

ADC conversion results from the register's CLEAR or RESET

values. At the same time, external decision-making based on

programmable thresholds (for example, Open-wire Detection),

must take the available V1ADC and VB1ADC code range into

account.

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