ADBMS2950BCCSZ datasheet(71/97 Pages) AD

71 / 97 page

Data Sheet

ADBMS2950B

Rev. 0 | Page 71 of 97

The polarity of the IxADC measurement results is different for

the two channels. With the recommended connection as

described above, channel I1 battery current is negative when

discharging (IxB > IxA) and positive when charging (IxA >

IxB); channel I2 inverse. The host controller software can invert

the measurement quantities as required.

The shunt resistor shorts all current sense inputs to GND,

which simplifies the input protection. Additionally, the

ADBMS2950B has internal ESD protection on all inputs as

shown in the Protection Features section. To further increase

protection, an external RC filter can be connected to the current

sense inputs as shown in Figure 51. The additional filtering is

not required for the IxADC inputs because of the preamplifier

having a low-pass filter characteristic. Still, the buffers in front

of the OCxADCs do not have this low-pass filter characteristic,

which allows to capture short current peaks through those fast

ADCs if required. On the other hand, the parasitic inductance

of the shunt resistor can cause an additional voltage drop during

fast current transients (di/dt), which could cause false

overcurrent measurements and false overcurrent alerts when a

short deglitch time configuration (OCDGT) is used. The

external RC filter can compensate for this effect and also reject

common mode signals.

RC Filter Selection

If the RC filter at the input matches the RL of the shunt (R × C

= L ÷ RSHUNT), the measured voltage follows the actual current in

an optimum way. The inductance of typical 50 μΩ to 100 μΩ

bus bar type shunts is on the order of 1 nH. The shunt

manufacturer can be consulted for measurement data if

required.

A simple recommendation is to set the time constant of the RC

filter to the maximum value that is still acceptable for

measuring the shortest current spikes to be detected by the

overcurrent ADCs. Alternatively, the RC filter can be adjusted

to yield the right response to a current step or ramp applied to

the shunt. Similar to the compensation that is done for scope

probes, but in typical cases the resulting time constants are

significantly shorter than what is required for fast overcurrent

detection within single-digit multiples of the OCxADC

conversion time, thus the first approach fits in typical use cases.

The series resistors can be up to 220 Ω per input; larger values

should be avoided as they could cause additional offset errors

due to pin leakage currents. While low resistor and high

capacitor values are preferred for measurement, resistor values

in the range from 120 Ω to 150 Ω are recommended for robust

diagnostics of the shunt connectivity.

The overcurrent detection deglitch time setting (OCDGT)

allows additional digital filtering to avoid false positive events in

the presence of current spikes. A good compromise for

detecting fast current events and filtering high frequency

signals is an RC setting of 120 Ω (Rp) and 220 nF (Cp) for all

common mode input filters.

Because of the single SxA pins per channel, differential filter

capacitors should not be added to the current and overcurrent

sense input pins as they lead to imbalanced filtering between

the Sx and Ix measurements and between the OC3ADC and the

other channels. On the other hand, a mismatch between the

common mode filters could cause a common mode to

differential conversion during AC signals. With the time

constant of the RC filter being very short, 26 µs for the

recommended RC values, and the mismatch between filters

being no more than 10%, the error is averaged out over the

IxADC conversion time.

An error signal could become visible during the shorter

OCxADC conversion time. Assuming a common mode step

input signal and 10% RC mismatch, the false differential signal

could reach 5% of the step as shown in following calculation.

Using the exponential equation describing the voltage at the

capacitor of the RC relative to the input signal:

VOUT ÷ VIN = 1 − e(−t÷τ), where τ = R × C

The relative output signal with different time constant after one

OCxADC conversion time becomes:

•

−10% RC error: RC = 23 µs: 1 − e(−62/23) = 93%

•

+10% RC error: RC = 29 µs: 1 − e(−62/29) = 88%

The resulting differential signal is 5% of the step. After three

OCxADC conversion times, the relative differential signal is

decreased to only 1%.

Common-mode input signals can be avoided by design and by

following the layout rules on the shunt's GND connection that

eliminates most signal on the IxA inputs as they are on the

GND side of the shunt. This ensures most of the differential

sense signal is seen on the IxB pins only. It significantly reduces

common mode to differential signal conversion of the values

calculated above. Additionally, it avoids OCxADC false trigger

events even with short deglitch time settings.

Current Sense Layout Recommendation

Figure 51 shows the recommended current sense input RC

filters. The placement and layout of the current sense input

circuitry should be symmetric on all inputs as shown in the

layout recommendation along the horizontal axis. This way,

temperature gradients that are more appropriate to appear

along the vertical axis do not cause differential thermocouple

voltage in the sense lines. Instead, all connection points (pad to

component, trace to via) are at the same temperature, and the

resulting thermocouple voltage that may develop is canceled

out. In that sense, it is also recommended to have any required

vias within a differential pair to be close together to ensure

good thermal coupling between them. For the same reason, any

connection points on the PCB should be at same temperature,

also the sense pads or pins of the shunt resistor should be at the

same temperature to minimize thermocouple voltages that lead

to offset errors. Symmetric heat dissipation of the shunt along

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