Dc voltage to analog current converter

[firstoption]

Good day to all,

Please i need your support . I am looking for an IC that i can use to design a DC Voltage to Analog current converter.The Analog current Signal should be in the range of 4mA to 20mA. I want to use the Analog current as an Input into to my Analog to Digital converter circuit . Any suggestions and idea will be highly appreciated. Thank you all for the usual Support.


Best regards.

 

[KlausST]

Hi,

the same question was some month ago.

the device is named "resistor".

No IC necessary.

Klaus

 

[ArticCynda]

Use a single transistor with a diode and 1k resistor between base and ground, and a resistor R between emitter and ground with value Ianalog = Vb / R.

 

[hobbyckts]

if you still need a device
https://www.digikey.com/product-sea...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

 

[firstoption]

Thank you all for taking the time to go through my post. I really appreciate your Kind gesture.Sir i would be very glad if somebody could try and explain this concept to me. Like klausST said above. I actually used a 250 Ohms resistor at the Input pin of my ADC circuit and i measured the current at that Point. The value of the current was 3.8mA when the DC voltage Output was 1V and 19.6mA when the voltage was 5V.This current range is okay for what i want to do but my Problem is that the Current Signal is a DC Signal, what i need is an Analog Signal. Because when i passed this current Signal into my ADC circuit,the Output i got for 1V was 58 while i had around 290 for 5V.That was not the result i was expecting to get. I was expecting to get the same 3.8mA for 1V and 19.6mA for 5V but now in digital value. I would be very grateful if somebody could put me through on what is going on. Once again thank you all for the Support. Best regards.

 

[hobbyckts]

How many bit ADC you are using? The ouptut from the ADC will be in HEX value only know you only need to convert those into appropriate voltage or current by using microcontroller or FPGA or Processor or whatever you are using

 

[chuckey]

I think you are mixing digital numbers up with analogue currents and voltages. - " i got for 1V was 58 while i had around 290 for 5V". This looks OK, five times the input signal give a five times higher digital number. The question is what do you want your digital number to be when the input, say 0V or 4 mA or what ever, and what you want the number to be at the other end of the range, 5 V or 20 mA. Just alter the feedback around the Opamp at the input of the ADC and its offset.
Frank

 

[firstoption]

Thank you once again for the response. I really appreciate your sense of understanding. I am actually using the ADC of Atmel Attiny87 microcontroller for the analog to digital conversion. The analog signal input to the ADC has a range of 4mA to 20mA,this range corresponds to 50ºC to 400ºC(that is 4mA=50ºC while 20mA=400ºC) . My final assignment is to convert the digital output signal from the ADC to the corresponding temperature.
I already have a Formular which I am using for the conversion.: TºC = (21.875 * x - 37.5); where x is the digital output value from the ADC. If x=4,the above equation will give 50 while it yields 400 when x=20 Once again thank you for the guide. Best regards.

 

[KlausST]

Hi,

It seems you are using VCC of the ATTiny as reference. If you need precision this often is no good idea.

Therefore i recommend to use an external VRef. Maybe 3.0V, 2.5V or less.
For further calculations let´s assume it to be 2.5V

theoretically you need a burden resistor of 2.5V/20mA.This gives 125 Ohms. Usually one doesn´t use the "limit" but gives some headroom.
Therefore i calculate with a resitor of 100 Ohms.

To limit noise you should use a passive low pass filter before the ADC input.
If you want to display the value then an update rate of 3 times per second should be the max display rate.

A first order filter (RC) with a cut off frequency of 1Hz ... 10Hz should filter out unwanted nois and thus stabilize temperature readings.
(Alternatively you could use oversampling and digital filtering. The benifit is that you could adjust filter parameters with software and gain some resolution (not precision))

The ADC input range now is 2.5V/100Ohms = 25mA.

50°C --> 4mA --> 400mV --> 164 LSB (with a 10 bit ADC)
400°C --> 20mA --> 2000mV --> 819 LSB

this means (819-164)LSB / (400-50)°C = 1.871 LSB/°C or 0.534 °C/LSB
and an offset of 70 LSB

so the new formula is T°C = 0.534 * (ADC - 70)

Klaus

 

[firstoption]

Thank you very very much Sir . I really appreciate your sense of understanding. I will make all the necessary adjustments in my design so as to make use of an external VRef. Once again thank you for taking the time to put me through. I really appreciate it. Best regards.