MM9Z1_638 Current Sampling

[Jon123]

Hello,

I am using the MM9Z1_638 to perform current sampling and I am encountering a problem:
I am using a 100μΩ current shunt to sample the current, and the PGA is set to 256.
When I sample currents below 20A and read the values of the ACQ_CURR0 and ACQ_CURR1 registers, the data is mostly normal.

Continuing to increase the current at this point, when the current exceeds 23.5A, for example, 25A, read the ACQ_CURR0 and ACQ_CURR1 register values and convert them to a current value of less than 100mA.

In this situation, how would you go about solving this problem?

Regards

MM9Z1_638 

[Q_man]

Can you share the schematics and layout of the connections between shunt and IC?

W.

[Jon123]

Thx, my problem has been solved！ My test and response had an error in the morning. At that time, the voltage between ISENSE_H/ISENSE_L and ground was 0.7V, which was 700mV. The reason was that my test harness was too long, causing a voltage drop of 700mV in high current conditions. This result caused my chip sampling pin to be 700mV with respect to the board ground.

I have solved the problem by shortening the harness. After retesting, the voltage between the sampling pin and ground is about 65mV at 30A. I have tested at gain of 256 and 64, and the maximum current that can be sampled is currently 35A. 

Additionally, I would like to ask, at a gain of 256, a voltage drop of 65mV does not meet the requirement of 50mV in the manual, will this have any other potential impact on my sampling?

Another point is whether my ISENSE_L/ISENSE_H must be connected to the GND of the circuit board. The current connection method is shown in the figure. The battery supplies power to the circuit board and the peripheral load at the same time, and the peripheral load circuit is connected in series with the sampling resistor, and the ISENSE_L is connected to the GND of the battery

[Q_man]

Hi,

it's difficult to know the reason, but I would first check that the connections between shunt and MM9Z1_638 are properly done.

Main point is that the measurement is GND referenced (typically on side of the shunt is connected to GND) and that the Vshunt+ and Vshunt- voltages are in line with this specification:

This means
- each pins voltage vs. GND should be less +/-50mV (gain 256)
- the voltage between both pins should be lower the +/-150mV.

In general I recommend to use automatic gain control, if not done already.

Rgds
W.

[Jon123]

Thank you for your reply. Based on your advice, I checked the voltage between ISENSEL/ISENSEH pins and ground on the circuit, and at the current gain of 256, the voltage between ISENSEL/ISENSEH pins and ground reached 50mV when the current exceeded 18V. When the current exceeded 25A, the MM9Z1638 could no longer collect data, and the voltage between the pins and ground reached 70mV. Additionally, I tried collecting data at a gain of 64 with a current of 25A, and the voltage between the pins and ground was also 70mV. The manual indicates that the range of voltage between the pins and ground at a gain of 64 is +/- 100mV, and the current reading was -700mA. If this is indeed the problem, how should I modify or adjust my code and circuit? Currently, the ISENSEL pin on the circuit should be directly connected to the power supply's GND.

[TomasVaverka]

Hi Wolfgang (@Q_man),

Could you please help with this question?

Thanks & BRs, Tomas