Part Number: INA250-Q1
We are trying to design a device for detecting a specific and configurable current value threshold, capable to generate a interruption in our MCU and wake up it.
(F1_GeneralSch) We have a 2-wires preassure sensor powered by 12v whcih provide a current value between 4-20 mA. Once our MCU has wake up, it must measure the current value on the line sensor so this current value MUST NOT be modified.
We are thinking on a detection system divided in 3 different states (F2_DetectionCircuitSch), but we dont know if you can provide a specific standard solution to cope with our porpuse directly, or cope with some of these different parts separatly.
Our first approximation (F3_DetectionCircuitSimulation)to the detection device is componed using ideal operational amplifiers: but we dont know how to select the correct elements and configuration values, because we had simulated it with ideal OA´s, but its possible unreach this features.
1- Using Rshut to convert current-voltage values without current sensor line modification. I=4mA -> Vrshut = 399.8mV || I=20mA -> Vrshut = 2v
2- We want adapt/convert Rshut voltage to a more suitable voltage range to make comparation easier (next stage), without reach saturation value on the OA.Vrshut=399.8mV -> Vout=1.2v || Vrshut=2v -> Vout = 6v || GAIN = 3.
We dont know if these 2 stages could be done using INA250-Q1.
3- Final stage made up with a comparator in which threshold value is set by Vref voltage (manuallyconfigurable using a potenciometer) . This stage is easiest because we have added the previous voltage adaptation stage.
For this part, we would use a LM293P or LM7301 with a voltage divider to set up Vref.
Do you have any integrated solution or aproximation to this device? Do you have any other circuit to cope with each stage separately?
It´s really important for us to minimize consumption on circuit elements.
Thank you for posting on the E2E Forums.
I would like to start by clarifying that the INA250 is not the best choice for the current level measurement required, given the low value of both RShunt and circuit currents. Furthermore, you seem to be refer to a schematic of your circuit, but I cannot find one attached. Would you be able to provide me with a few more details?
As far as I understand you have a low power setup where the sensor is permanently connected and upon reaching out a specific output threshold, it should wake up the MCU so as to perform additional tasks. I am not clear if this should be powered from the 4-20mA loop or whenever this loop is actually powered by the same supply to the MCU, what are the available voltage rails, if there is already a sense resistor in the circuit and what is the value for such.
For the best linearity, I suggest the INA200 or the more versatile INA301, both with a gain of 20, as possible solutions. The latter can perform the comparison by directly comparing the output with a given reference voltage obtained from a potentiometer, DAC or resistor, which can be interfaced with the mcu, both as a analog voltage (376 to 1880mV for a sense resistor of value = 4.7R or 800 to 4000mV for a sense resistor of value = 10R) and as a digital output to trigger an interrupt.
Carlos S., TI Sensing Products, Applications Support
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In reply to Carlos Silva85:
Thank you for your fast answer.
Sorry for the inconviniences, but I try to attach schematics files and I failed.
With these files you should have answers to doubts about how the current loop sensor is, and our initial approach to solution using operational amplifiers.
We have a sensor line powered with 12v (external battery), and the sensor stack current level (4-20mA due to preassure level) on that sensor line. With this battery, we have a separately power line to other circuit of devices.
A really important specification is voltage reference to thresshold due to this Vref should be manually set up using a potenciometer (by user) and voltimeter, so output values from "current-voltage conversion stage" must be enough to comparator system (or must be asjusted previously).
We are going to check features or both INA200 and INA301 that you recommend, but we think there are some problems with them:
INA200 comparator have a Vref fixed and we need to be able to modify it; and INA301 have a limited range of power supply values and we need to power with 12v without voltage regulator.
Thank you for your help
In reply to Adrian Cobos:
Thank your for providing additional information/
These are my views on your circuit:
The INA139 or 169 can be supplied directly without a regulator, nevertheless care should be taken to make sure there is no noise on the supply voltage.
It´s a really good idea to use regulate power on the circuit, specially on the reference voltage to avoid unexpected performance. Thank you for information about OA´s hysteresis, we want to include it once current-voltage conversion is finished.
We think INA139/169 have the following problem (F4_CurrentDivision): when the OA polarize transistor base, transistor drains current from colector pin which comes from sensor line, so current on this line will be divided between main line and chip line (as we try to show on the red circle on the picture), therefore when MCU read current value on sensor line (4-20mA), it has been varied.
Unfortunately all our parts have some input bias current, therefore that cannot be avoided.
There are, however, parts with different bias currents. The INA139 has a bias current of only 10uA (the schematic is merely representative), when set with a common mode voltage of 12V and a gain of 25. The gain is set with an output resistor, so effectively using a voltage divider it is possible to attenuate this to whatever gain is required, while keeping the bias current at the spec'd 10uA, which only adds 0.25% error with respect to the minimum 4mA loop current.
Would this be acceptable? This, of course, assumes a common mode voltage of 12V. I don't know where the current measurement is taking place. Is the sense resistor directly grounded or does the device 'tap' the line, requiring a differential measurement? If the latter, is the differential voltage greater than 12V?
We were checking this bias current values and it´s true that is impossible to avoid it, but not only in your device.
With configuration settings that you recommend, we could have a acceptable range on output values, and if the bias current, more or less, keep 10uA near to 12v power supply in the whole range of 4-20mA we can manage to this value and right it after measurement process. Then, we are adquire INA139 chip and check features in a evaluation board into our system.
Thank you for all your help, we appreciate it a lot.
PD: If you want, you can mark this tip as "solved"
Great to know. Does your 4-20mA loop originates from the 12V supply as well?
I have some testing to do this week with an INA139 and can certainly build the circuit and take some measurements for you to use as a reference.
I will come back once I have these to share.
As promised I run some tests. Since you haven't mentioned the common mode voltages required for the circuit I decided to evaluate a few different devices and play safe on my recommendation as to cover all your requirements.
I recommend the INA199 A1 with the circuit below to go with your required output voltage/gain. Note that the minimum supply voltage to the device is 11V for correct operation.
The measurements for the input bias current are as follows:
20.012mA - Output = 9.98V
170uA = 0.85% Error
3.998mA - Output = 1.99V
55uA = 1.4% Error
The measurements above are based on high side sensing and the error is a bit smaller when a low side measurement is taken.
I have used a voltage divider to obtain the voltage levels required, the 1.5K resistor can be replaced with a potentiometer of the same value to adjust the value at which the comparator is triggered.
Thank you for make some test evaluation about consumption. We appreciate it a lot.
Our power line (12v) provide enegy voltage to the whole circuit (all different parts and devices, and the sensor current line), therefore you can power INA199-A1 and voltage value is the same which sensor line is.
We think INA-199A1 is valid too for our porpuse (and we think that fit better than INA139, although fuctionality is really similar) and bias current consumption are really acceptable. We´ll buy some samples of both components and try that features.
Thank you for your time.
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