OPA322: Some problems about OPA322 and FDC2214EVM

I see three issues:

1. In the way you connect the two batteries to the board, you supply the whole circuit single supply. This is not what the first schematic is showing.

2. The printed circuit board should be cleaned after soldering. Take some alcohol and a q-tip.

3. The supply voltage decoupling cap at the OPA320 is missing!

Kai

Thanks for reply.

1. There are 2 output on this board.One is a positive output voltage, one is a negative output voltage, GND in the middle. There is no problem with the connection in my drawing. I have measured it with a multimeter. The voltage are +2.496V and -2.496V, respectively.Is  it  not consistent with the first schematic?

2. Thanks to your notice. And does this way you mentioned help to solve my problem?

3. Could you please explain it more specifically? Or  can you tell me where others have explained it?

Regards.

As you haven't connected the mid-point of the two batteries to anything, the circuit only sees single supply

Kai

Hello, 

As Kai points out, looking at your physical connections it would seem that you have put batteries in series (for a total supply of 5V). When you have measured your supply voltages to what ground reference are you using? I imagine between your batteries (where the red and black wire meet). You will want to establish a ground connection to the EVM to have split supply. 

All the best,
Carolina

Kai

I changed the connection as shown in the picture below.I connected the original series connection point of the two batteries to the GND interface.Is that what you're talking about?

The complete circuit wiring is shown below.I have made sure there is no connection error.

One of the voltage regulator outputs is +2.498V and the other is -2.498V. Both outputs are based on the middle GND.
As shown in the following two figures.
(The top two outputs are positive, the middle two GND, and the bottom two are negative.)

Connect the above two output voltages to V+ and V- ports of OPA322, respectively.

Connect the + In terminal of OPA322 to the positive terminal of the lithium battery, and then measure the voltage difference between the + In terminal and the negative terminal of the battery, that is, the voltage difference between the two points circled IN the following figure is +3.742V.

Finally, the voltage difference between the -in /OUT port and the cathode of the battery, which is the voltage difference indicated IN the figure, is measured at +3.232V.
It is not the theoretical +3.742V.

Is this an acceptable outcome?
I want to use this to connect to my shielded electrodes.

Regards.

Thanks for response,

I replied to Kai above.I described the circuit modification I made and the whole experiment process above.

But the  results didn't change.Could you please help to have a look, and then help to analyze the reasons and solutions?

Regards.

What is the board with the LM317/LM337? Where does it come from? How do you want to use it with the OPA322? What EVM do you want to use? How do you want to connect all these things?

Can you draw a schematic or at least a sketch?

Kai

1，What is the board with the LM317/LM337?  Where does it come from? How do you want to use it with the OPA322? 

This is a voltage regulation module, my purpose is to use this board to get +/-2.5V voltage for OPA322. 

2.What EVM do you want to use? How do you want to connect all these things?Can you draw a schematic or at least a sketch?

FDC2214EVM.  I want to connect it to a copper plate for use as a shielding electrode.

I have given the schematic diagram at the beginning, as shown below.

Regards.

Hey, 

Did you try probing the output with an oscilloscope, can you share scope captures? 

All the best,
Carolina

No, I didn't.

I only used a multimeter to measure DC voltage.

If necessary, I can use an oscilloscope to capture the waveform.

regards.

Hello, 

Yes I wonder if the multimeter is sinking some current and not providing an accurate output. Or if there is ringing by the load you are driving (the shield), a oscope might show a more clear picture. 

Also I noticed from your original picture from 7/12 is not the same connection as your pictures from 7/13. 
I thought the blue batteries were used to power the op amp with ±2.5V, but in the picture from 7/13 it would seem it is being used to power the EVM board and the op amp is being powered by the grey battery. 

When I mentioned to connect the connection between red and black (GND between dual supplies) should be connected to the EVM, it was to make all the grounds the same. It looks like this is lost again when the op amp is powered by the grey battery. 

All the best,
Carolina 

As Caro already mentioned, the GND connection to the FDC2214 EVM is missing:

A better way is to run the 0V wire to the OPA322 board, together with the +/-2.5V wires and then from this board to the FDC2214 EVM. This allows you to add 100nF decoupling caps from the supply voltage pins of OPA322 to 0V. Use shortest connections for the two 100nF caps. Also see figure 41 of datasheet of OPA322.

Kai

Thanks.

A better way is to run the 0V wire to the OPA322 board, together with the +/-2.5V wires and then from this board to the FDC2214 EVM.

But there is no GND port on my OPA322 board. I can only connect the  GND to the FDC2214 EVM .

Regards.

Then create a GND port

The supply voltage decoupling for the OPA322 -which you missed up to now- is an urgent part of the circuit and needs this GND port. So create a GND port. Route GND from the +/-2.5V supply voltage generation to the OPA322 board and from this board to the FDC2214EVM.

The input of OPA322 also needs this GND connection, as path for the input bias current.

So there are two gound reasons for adding a GND port on the OPA322 board.

Finally, keep in mind that the OPA322 is a HF OPAmp. Therefore, consequently follow HF design practises.

Kai

Thanks a lot.

Is it ok if I follow the circuit structure shown below?

Regards.

Yes, looks good.

Kai

Thanks.

In my circuit diagram above, setting both capacitors C to 20pF, do you think this is ok?

Or do you have a more recommended capacitance value?

Regards.

20pF is tooooooooooooooooo small. 100nF/X7R is a good choice.

See also section 7.3.1 of datasheet of OPA322.

Kai

Thanks so much.

Hello,Kai.

I'm sorry to bother you again.Some things delayed the process of the experiment, so I started to do the experiment again recently.

I did the experiment according to the method you said before, but it still failed to achieve the desired effect.

I added two capacitors and GND ports and remade the PCB board.

Then wire in the original way and connect the GND on the two components together.

But despite this operation, it is not possible to get the INPUT/output of OPA322.

In the diagram, the input voltage of the amplifier is about 3.990V, but the output voltage is less than 2.5000V.

Regards.

Hello,

It is rather hard to follow the wiring through the pictures, can you please draw out the connections? Preferably through a software like TINA but paper is also fine. 

All the best,
Carolina 

If you power the OPA322 with +/-2.5V, then you must not give +3.99V to the input. As the input voltage being higher than the supply voltage, this will destroy the OPA322.

Also, as being powered by +/-2.5V, the output voltage of OPA322 can never become higher than 2.5V.

Best you take a fresh OPA322 and try again. But please with an input voltage <2.5V!!

Kai

Thanks so much.

I'll try again with a different input.

Regards.

I'm sorry that I can only draw by hand because I can't use Tina.

The circuit connections are shown in the figure.

Regards.