Hi All,
I hope that I am posting to the right forum. After days of searching, reading, experimenting, I still have not been able to find a proper solution which I can tailor to my issue, so it is time to ask you guys.
I made a voltmeter which consists of an Arduino microcontroller and an ADS1115 module that I use in differential mode because I measure negative voltages too. Unfortunately, this ADC module does not have high enough input impedance for the task I want to perform, so it influences the measured circuit. I thought I should build a voltage follower using an op-amp with a high enough input impedance and connect it to the inputs of the ADC.
So, I found this TL072 op-amp and I built a voltage follower with it. Here is an ugly sketch of the relevant part of the follower:
The input (A) takes the let's say positive input and ''forwards" it to the A0 input of the ADS1115 module. The input (B) does the same thing but it goes to A1. The measured voltage range is roughly +/- 3 V, or even lower. The +/-15 V power is supplied by a XL6007e1-based power supply circuit which is a +/- power supply with a step up converter, so I can power it from the microcontroller or USB directly.
So, after the long introduction, let me tell the issue. First of all, the circuit works very well when the inputs of the op-amp are connected to a voltage source. However, when I remove all the connections from the input (A and B are floating), the op-amp goes bananas and it starts to overheat. Within a minute, I cannot hold my finger on the chip anymore. Furthermore, I could measure ~33 mA current flowing between the output of the op-amp and the input of the ADC which is not good at all. Interestingly, when I do not connect the outputs of the op-amp to the AD converter, despite the fact that I let the inputs of the float, the op-amp does not overheat! So, I am extremely confused with this issue.
Finally, my questions:
1.) Is there any explanation/solution for this issue? Could it be that the 3 kOhm resistor is too small, or am I overlooking something trivial?
2.) Are there any better op-amps than this one for voltage-follower purposes within the range of giga/terra-Ohm input impedance?
Thank you very much for your time!
I hope that I am posting to the right forum. After days of searching, reading, experimenting, I still have not been able to find a proper solution which I can tailor to my issue, so it is time to ask you guys.
I made a voltmeter which consists of an Arduino microcontroller and an ADS1115 module that I use in differential mode because I measure negative voltages too. Unfortunately, this ADC module does not have high enough input impedance for the task I want to perform, so it influences the measured circuit. I thought I should build a voltage follower using an op-amp with a high enough input impedance and connect it to the inputs of the ADC.
So, I found this TL072 op-amp and I built a voltage follower with it. Here is an ugly sketch of the relevant part of the follower:
The input (A) takes the let's say positive input and ''forwards" it to the A0 input of the ADS1115 module. The input (B) does the same thing but it goes to A1. The measured voltage range is roughly +/- 3 V, or even lower. The +/-15 V power is supplied by a XL6007e1-based power supply circuit which is a +/- power supply with a step up converter, so I can power it from the microcontroller or USB directly.
So, after the long introduction, let me tell the issue. First of all, the circuit works very well when the inputs of the op-amp are connected to a voltage source. However, when I remove all the connections from the input (A and B are floating), the op-amp goes bananas and it starts to overheat. Within a minute, I cannot hold my finger on the chip anymore. Furthermore, I could measure ~33 mA current flowing between the output of the op-amp and the input of the ADC which is not good at all. Interestingly, when I do not connect the outputs of the op-amp to the AD converter, despite the fact that I let the inputs of the float, the op-amp does not overheat! So, I am extremely confused with this issue.
Finally, my questions:
1.) Is there any explanation/solution for this issue? Could it be that the 3 kOhm resistor is too small, or am I overlooking something trivial?
2.) Are there any better op-amps than this one for voltage-follower purposes within the range of giga/terra-Ohm input impedance?
Thank you very much for your time!