Experts who are familiar with 7135 are welcome to explain and discuss if interested.

呜呼哀哉

Experts who are familiar with 7135 are welcome to explain and discuss if interested. [Copy link]

The internal block diagram of the ADC ICL7135 is as follows. After the A/Z process closes the internal circuit, the circuit diagram is equal to the pencil drawing (I drew it myself, there may be some mistakes, please correct me if there are any mistakes)

Now I don't understand how the A/Z process eliminates the offset voltage of the op amp. The integration circuit is a resistor and a capacitor, and the connection point is connected to the inverting input terminal. Now the connection point is connected to the capacitor, and the feedback is equal to no. How should this circuit be derived? Thank you all.

maychang

"After the A/Z process closes the internal circuit, the circuit diagram is equal to the pencil drawing"

The "pencil drawing" is missing. Please add it.

呜呼哀哉

maychang posted on 2021-9-1 11:47 "The circuit diagram after the A/Z process closes the internal circuit is equal to the pencil drawing" No "pencil drawing" was found. Please add it.

Sorry, I forgot to click on the image upload. The inverting input of the integrator should be grounded, but the symbol is missing.

maychang

Alas, posted on 2021-9-1 13:39 maychang posted on 2021-9-1 11:47 "The circuit diagram after the A/Z process closes the internal circuit is equal to the pencil drawing" No "pencil drawing" was seen...

"The inverting input of the integrator should be grounded, but the symbol is missing."

After figuring out that the inverting input of the integrator is grounded, the first question, "I don't understand how the A/Z process eliminates the offset voltage of the op amp", is now clear? The A/Z process stores the offset voltage of the op amp on the C AZ capacitor, which just offsets the error generated in the integration process.

maychang

Alas, posted on 2021-9-1 13:39 maychang posted on 2021-9-1 11:47 "The circuit diagram after the A/Z process closes the internal circuit is equal to the pencil drawing" No "pencil drawing" was seen...

This is a very clever design. The integration capacitor C INT and the integration resistor R INT do not need to be accurate, and the temperature coefficient of the capacitor and resistor do not need to be considered, as long as the capacitance and resistance values do not change during a conversion. The offset of the op amp does not need to be considered. During the A/Z process, the offset voltage of the op amp is stored on the capacitor C AZ , which automatically offsets the error caused by the op amp offset during the integration process.

呜呼哀哉

maychang posted on 2021-9-1 16:31 "The inverting input of the integrator should be grounded, and this symbol is lost. " After figuring out the grounding of the inverting input of the integrator, the first floor's question "What to do...

Yes, after the inverting input of the integrator is grounded, the offset voltage of the buffer buff in front is indeed loaded on Caz. At this time, the integrator, both in the same phase and in the opposite direction, is actually still an open loop. Will the output of the integrator be the offset voltage? What about the voltage on Cint?

In the next stage, the integration process, the buffer is connected to the positive input and the negative input is GND, which is equivalent to the integrator's non-inverting end being grounded and the inverting end being connected to Caz, and a closed loop cannot be formed (the Caz position of the standard integrator should be short-circuited). How should the circuit be analyzed at this time?

maychang

Alas, published on 2021-9-1 16:53 Yes, after the inverting input of the integrator is grounded, the offset voltage of the buffer buff in front is indeed loaded on Caz. At this time, the integrator, in the same phase and in the opposite direction are both connected...

This is a long story.

The second picture on the first floor is copied from the datasheet of ICL7135. I believe you have this datasheet in your hand. The datasheet describes it in great detail: there are four stages in one conversion, the automatic zeroing stage, the forward integration stage, the reverse integration stage, and the output stage. As long as you understand the circuit operation of the four stages, the principle of automatic offset cancellation of the op amp is easy to understand.

Because the content is large and the length of the forum post is limited, you may wish to read the datasheet carefully first. If you still don't understand after reading it carefully, then ask questions in the forum. This forum is full of experts and everyone will help you.

呜呼哀哉

maychang posted on 2021-9-1 19:21 This is a long story. The second picture on the first floor was copied from the ICL7135 datasheet. I guess you have this datasheet in your hand. ...

This device is too old and probably few people know about it. You are the only one who has answered this question in such a long time.

I have read the datasheet many times, checked the dual-slope integrating ADC application guide and other things, and made the product.

500V, 200M insulation meter.

500V, 2000M insulation meter.

The absolute error accuracy is only 1,2M. I just can't figure out that part of the op amp circuit, which subverts my understanding of op amps and makes me itchy.

And the parameters calculated according to some contents in the datasheet are incorrect.

okhxyyo

Alas, posted on 2021-9-2 08:51 maychang posted on 2021-9-1 19:21 This is a long story. The second picture on the first floor was copied from the ICL7135 datasheet...

Are you using it in product development? If the device is too old, are you considering looking for a replacement in case the device is no longer produced?

呜呼哀哉

okhxyyo posted on 2021-9-2 09:23 Are you using it in product development? ? If the device is too old, consider finding a replacement to prevent the device from being discontinued later.

This is not a problem. TI, Renesas, and several other companies are producing it, and judging from their official websites, there is no indication of imminent discontinuation of production.

maychang

Alas, posted on 2021-9-2 08:51 maychang posted on 2021-9-1 19:21 This is a long story. The second picture on the first floor was copied from the ICL7135 datasheet...

This type of dual-integral AD conversion chip is indeed very old.

This type of chip has a very low conversion speed, generally only a few times per second, but it is low in price and has good conversion accuracy, so it is more suitable for use in visual digital multimeters, voltmeters, etc.