Regarding current sense

[NJani.2]

Hello Support,

I have application in wjhich i have to sense current for negative supply. specifications are as below.

Negative supply voltage range( at RS- pin) : -5 V to -2 V

Current range that needs to be sensed: 0.1 uA to 20uA

The output of OPAMP needs to be read by 10 bit ADC of controller. 

we have one application note AN4451 in which it shows nA range. but in our application the voltage is negative so please guide regarding that or suggest some part number.

Thanks

[AUPETIT nicolas]

​Hello

We can firstly have a look on two different way to measure a current with a negative common mode voltage.

The simpler is to use a current sensing with an integrated gain able to work in this manner. It can be our TSC2012 product which accept a Vicm until -20V. It has a gain of 100 which is recommended to measure small current.

In order to go deeper in the analysis, it would necessary to have more information about your application as the power supply , the shunt value you have selected and also the precison you are expecting on the output of the current sensing (especially for the lowest current to sense 100nA).

The second option is to use a precise Opamp as the TSZ121. But the convenience with this option is a classical opamp cannot accept a negative Vicm. To use it in such condition you have to power your opamp with neagtive supply as well.

For example put -5V on the GND pin and 0V on Vcc pin. In this manner your opamp is powered in 5V and can accept the Vicm=-5V. But take care the ouptut of your opamp will in the range of [-5V and 0V]. Is your ADC can accept such Voltage range? Otherwise you can have a look to the application note in attachement.

A last option, if possible, is to sense the current in low side (between the load and the Ground).

In this case you are not anymore constraint by a negative Vicm, as this one become Gnd.

And you can use an Opamp as the TSZ121 powered with a 5V for example, or if you need an integrated gain for more integration and precision you can also think about the TSC212 current sensing.

Please let me know which of this 3 options seem more suitable for you, or if something is not clear , and give me more details about your application to go in a deeper analysis.

Best regards

[NJani.2]

Thank you so much for your prompt reply.

I will explain detailed application.

I have a requirement of gate current sensing for the high power amplifier. The current variation is between 0.1 uA to 20 uA.

I have power amplifier as load which will draw 0.1uA current and in case of any problem occur such as shorting it will draw 20uA current so i have to continue sense current for knowing device condition.

input voltage of my power amplifier is -5V in initial stage and in working condition voltage is -2V, in both draw current is almost same in 1.4uA. 

Input voltage -5V to -2V negative supply and it switching is in micro second.

Below are the application details asked from you side.

The power supply will be in the range of -5 V to -2 V

For Rshunt value selection, we are considering the below calculation.

For 0.1 uA current, Voltage drop across Rshunt needs to higher than input offset voltage, right? as our current range is lower in uA. it is ok for us to have higher Rshunt resistor. kindly suggest the suitable Rshunt.

we are expecting highest possible precision on the output of the current sensing for lower current 0.1 uA to 5 uA. as in normal condition , our high power amplifier gate will draw the lowest uA current.

Please guide and suggest best suitable option as per above explanation.

Thanks

[NJani.2]

Hello,

Waiting for your response. Please guide.

Thanks

[AUPETIT nicolas]

hello

Just to be sure to understand correctly your point about power supply.

Do you mean the current sensing and also the ADC will be powered with a negative power supply as well in the range of [-5V:-2V]?

like the principle schematic attached.

It also mean that the current sensing and the ADC must able to work with a minimum power supply of -2V? is it right?

What does mean for you highest possible precision for the lowest current 1%, 5% 10%? please precise me a range to evaluate the shunt, the gain of the current sensing?

From your previous description it seems low side is not possible for your application is it right?

At first order we can imagine to use a TSZ121 which is a precise amplifier with a maximum Vio of 8µV and can work with a power supply until 1.8V, as proposed in the second option

Please let me know the above information in order to propose the most accurate solution, in term of gain shunt and power supply and archi.

BR

[NJani.2]

Hi,

I think there is some misunderstanding.

Kindly find attached herewith diagram what we have assumed for current sense implementation.

Yes may be low side current sense is not possible in our application but we want to confirm from your side.

precision required is 1% or 5%.

ADC input voltage range is 0 to 3.3 V , so we are planning to use voltage divider at the output of op-amp to shift the negative voltage to ADC input voltage range.

Please guide.

[AUPETIT nicolas]

Thanks for the clarification

Nevertheless let me still clarify this point:

Yes may be low side current sense is not possible in our application but we want to confirm from your side.

A low side configuration is the simplest configuration as you can directly address the ADC, and the best one in term of precision,

Just let me propose a low side configuration with a first order approach of expected error, and let me know if it possible for you to implement it

With a shunt of 10kohms, the voltage to amplify will be in the range of 1mV to 200mV. With a gain of 16 the theoretical output will be in the range of 16mV to 3.2V

 

In the attached schematic, the Vicm is fixed to GND, so there is no variation of the input offset due to Vicm variation

Thanks to low side configuration, the CMRR due to the external resistance is null.

To simplify let consider only the input offset voltage and the resistance precision.

The resistances (gain and shunt) have a precision of 0.1%, so a gain error of 02% can be expected + 0.1% of shunt precision, so the external resistance add 0.3% error.

Then considering only the Vio of 8µV (maximum over temperature) for the lowest current (100nA) it add in worst condition 0.8% error.

So at first order 1.1% of error can be expected for the lowest current to measure.

One of the main draw back of a low side configuration that it is difficult to detect a short cut to the ground.

So just tell if this kind of implantation can be suitable for your application or not. Then we can go deeper in the analysis, or only consider high side configuration as previous schematic with an Opamp able to work with 10V power supply.

thanks

BR

[NJani.2]

Hi,

Thank your detailed guiddance.

As per our understanding we want to use high side because my power amplifier(LOAD) is getting directly connected with ground with that configuration. if we go for the low side, then the power amplifier( load) will get indirect ground via shunt resistor of 10K ohm, which may cause any damage to load.

If you confirm, that low side configuration and indirect grounding to load is not an issue, then we are good to go with low side configuration.

If not , then we need to go with high side.

Please confirm.

For Rshunt, we are ok with the 10K shunt ohm , as our load current is in uA range.

[NJani.2]

Hello,

Waiting for your response.. Please guide.

Thanks

[AUPETIT nicolas]

Hello

The shunt connected between load and ground can have an impact , if there is large current spike, that may perturbate the GND.

Note that the same thing can happen with High side configuration, large current spike will impact the supply of the power amplifier.

Anyway please have a look on the attached document, which described a high side configuration, and maximum error that can be expected on output.

it is a worst case calculus considering max value and max temp.

In this configuration, it is used a 16V amplifier powered between -5V and +5V. The most precise amplifier available in our portfolio, is the TSX711A.

https://www.st.com/en/amplifiers-and-comparators/tsx711.html

As you can see on the attached document due to its Vio=350µV max over temperature, and keeping a shunt of 10kohms the accuracy of 1% or even 5%, cannot be achieved for the lowest current (100nA).

You can increase for sure the shunt resistance value (and so decrease the gain) , but the next issue will be the voltage drop into the shunt. It will become higher and then affect the power supply of the power amplifier. Power dissipation will also increase.

On top of that due to the high side configuration, the external resistance (use for the gain), will add an error (CMRRes) principally when the power supply or Vp will be at -2V. In this calculus 0.1% resistance precision have been used.

So to achieve the precision you need, the best solution is low side current sensing approach by using the TSZ121.

https://www.st.com/en/amplifiers-and-comparators/tsz121.html#cad-resources

Some trial must be done directly in your application in order to check if it is possible.

A first approach can also to realize some simulation of your environment using our Opamp model that you can find here for the TSZ121:

https://www.st.com/en/amplifiers-and-comparators/tsz121.html#cad-resources

and here for the TSX711A:

https://www.st.com/en/amplifiers-and-comparators/tsx711.html#cad-resources

The consumption of few µA seems very low for a power amplifier. Note that the example of gain shunt... have been done for a range of current (100nA to 20µA).

BR