# DAC + op-amp buffer just keeps dying

I am using MCP4728 DAC which I am controlling nicely from the microcontroller. On its output I have connected the op-amp in buffer configuration(non-inverting input is the signal from DAC, inverting input and output are connected together), all powered with 5V on positive rail and gnd on negative rail. Op-amp part is AZV321(rail-to-rail).

Output of buffer op-amp goes through two more stages of amplification which end up in the base of darlington transistor. So, a little current is needed from the buffer.

What happens in my case is that when I connect the circuit, only the connected DAC channel stops working - it doesn't give any voltage out anymore.

I find it hard to understand what happens. From my understanding, op-amp is correctly connected and there shouldn't be any problems. I have tried switching DAC & op-amp, in case I have soldered then wrong.

• add a schematic. Without an exact schematic, we really can't even guess any better than you can. – Marcus Müller Oct 31 '16 at 22:22
• AZV321 has rail-to-rail outputs, but not rail-to-rail inputs. Maximum input voltage is 4.2 V with a 5 V supply. – The Photon Oct 31 '16 at 22:35
• Sorry! Added the schematics. -- maximum that DAC can output is 4.096V so no problem about input voltage to the op-amp. – davaradijator Oct 31 '16 at 23:30
• "only the connected DAC channel stops working - it doesn't give any voltage out anymore." Well, sure it does, even if the voltage is zero. And exactly which signal is misbehaving - DAC1, DAC1*, or V CH1? Do you have a load on V CH1, and if so what is it? When your problem occurs, what are the voltages on the other signals I've mentioned? – WhatRoughBeast Nov 1 '16 at 2:57

The DAC has on-board EEPROM and I'll place a small bet that somehow you have corrupted this memory and caused the output switches in the DAC to go open circuit and remain open circuit: -

The DAC input codes, device configuration bits, and I$^2$C address bits are programmable to the nonvolatile memory (EEPROM) by using I$^2$C serial interface commands. The nonvolatile memory feature enables the DAC device to hold the DAC input codes during power-off time, allowing the DAC outputs to be available immediately after power-up with the saved settings. This feature is very useful when the DAC device is used as a supporting device for other devices in the application’s network.

It's likely that to resolve this you should try reconfiguring the DAC via the I$_2$C from scratch. As for the reason, it could be bad layout or poor power supply decoupling or a dodgy power supply.

I'm have a hard time interpreting your question. Could you clarify the following?

What happens in my case is that when I connect the circuit, only the connected DAC channel stops working - it doesn't give any voltage out anymore.

From the DAC Datasheet:

The DAC outputs voltage, so you can identify your problem by building your circuit in steps.
1. Set up DAC. Is it outputing the voltage you expect?
2. Set up op amp follower. Operating as expected?
3. Set up stage 2,
4. Set up stage 3,
5. Attach transistor.

This will help you localize you problem to a circuit component or sub-circuit. Finally, if your singals are <10MHz, you might want to consider building this on a breadboard before soldering it.

• Thanks for your answer. What I wanted to say is that there is total of 4 DAC outputs and I am using only one for my purpose. That specific DAC output I am using stops permanently working after connecting to my circuit. My circuit doesn't work as expected at the point 2. So DAC gives voltage I expect, but the op-amp doesn't follow - it doesn't give anything at its output. I am guessing that I have picked the wrong part for my usage? – davaradijator Oct 31 '16 at 23:35
• Test your buffer circuit on its own. Just make a voltage divider – Edgar P Nov 1 '16 at 5:39
• Test your buffer circuit on its own. Just make a voltage divider to the input and check your output. It sounds like you are sinking too much current from the DAC, but its hard to tell. – Edgar P Nov 1 '16 at 5:48
• How is possible that I am sinking too much current if I am using buffer op-amp? Isn't the op-amp buffer taking just a very little current? – davaradijator Nov 1 '16 at 7:35
• That would be the case if the circuit were working correctly. It looks like you did a fine job selecting the components (from a short glance). I'm suggesting you look for a current sink that might not allow the DAC to develop a voltage on the line. This could be a short on the board from a bad solder or some other load on the line as another commenter mentioned. – Edgar P Nov 1 '16 at 8:39