INA851: Driving ADS124S08 with INA851

Hello Bekir,

I have a series of questions that will help us design the proposed circuit.

Shunt Questions

1. Shunt placement: Are you looking to do a low side current sense, or a high side current sense?
   1. If this is high side sensing, what is the voltage of your load supply rail, note that the INA must have a supply rail above the load supply voltage to work properly for high side sensing.
2. What size shunt are you using (how much power loss through the shunt is acceptable)
3. What is the range of current you are sensing?

INA Questions

1. What supply rails are available? Depending on high side vs low side sense, you may need to adjust these. If you are doing low side sense, symmetrical split supplies will be best.
2. Are you sensing unidirectional current or bidirectional current?
3. How fast of a change in current do you need to sense? This will determine the cutoff of the input filters to the INA851.

ADC Questions

1. Do you need all 4 channels to be sensed simultaneously? The ADC you have selected has multiplexed inputs, so only one input differential pair will be active at a time
2. How many samples/sec do you need? (alternatively, what is your equivalent number of bits)
3. What supply voltage are you using the ADC at?

Best,
Gerasimos

Hello Gerasimos,

We design a current sensing system for four solar cells. Each solar cell acts as a self-biased current source. A 100 mΩ precision shunt resistor is placed directly across each solar cell. The sensing configuration is floating differential sensing. The shunt is not connected to system ground. The sensed current is unidirectional because the solar cell only generates current in one direction (when exposed to light).

The expected current range is 0 to 800 mA. At maximum current, the voltage drop across the shunt is 80 mV. This range will map to an irradiance range of 0 to 1600 W/m2. The power dissipation is about 64 mW, which is acceptable for this application.

The system does not require a fast response time. The irradiance measured from all 4 solar cells is sent once per second to the receiver via RS485 using MODBUS RTU protocol.

The supply rails can be configured depending on the components selected. We are currently considering ±2.5 V discrete supplies for the ADC. Since simultaneous sampling is not required, a multiplexed ADC is acceptable. We prefer to use an ADC with 16 or more bits to achieve high resolution and precise measurement results.

Hello Bekir,

Sorry for the delay here. Depending on the common-mode each signal is at you may need to add a difference amp as a front-end to bring the voltages back into an appropriate domain. INA149 was designed specifically for this.

Similar in concept to the above, but instead of placing the inputs of the INA across the solar cell, they would be across the shunt resistor at each solar cell. Each INA149 will reject the common-mode voltage of each panel, and then pass just the difference as a low-voltage output.

A supply rail of +/-2. 5V allows for a good selection of devices, just make sure to be within a valid common-mode range for your device. However, you will not be able to use t he INA851, as it requires a minimum supply voltage of +/-4V for operation.

Since the high voltage common-mode portion of the signal can be removed by the INA149, you may be able to use the INA333, which is a low-voltage instrumentation amplifier. Unfortunately, we do not have low voltage differential in, differential out instrumentation amps, but you may use a pseudo-differential input to your ADC.

If the differential input to the ADC is needed, you may need to provide additional supply rails to accommodate the INA851.

Best,
Gerasimos