He said that the frequency is variable and up to 2 Mhz, the range of frequency hasn't answer yet.

You can take a dual variable capacitor to adjust the frequency.

 

See

 

You can take a dual variable capacitor to adjust the frequency.

Why you suggested to using vc(variable capacitor) to adjust the frequency, don't you afraid of the frequency range will be too narrow to adjust, we normally use the VR to adjust.

Could you upload the .asc file in #22.

 

Hi,

Well, the only difference is that a low level triangle is added to the normal ADC input which is the input to be measured, and the output of the summer goes to the ADC. It's that simple of a block diagram.
So nothing changes but we add:
1. A low level triangle.
2. A summer, such as with an op amp.

If you need to use the uC then it looks more complicated, and how is the Vin?

 

Upload Draft558.zip (#22)

 

If you need to use the uC then it looks more complicated, and how is the Vin?

Hi again,

Well, that's a dithered ADC input for ya
We need a uC and that is usually part of the ADC anyway, or should i say that the ADC is part of the uC anyway. The uC code does the oversampling and i dont see any way around that, but then again when we decide to oversample we are going to have an uC already anyway or something similar. An Arduino perhaps?

 

I believe that the inverting combiner has a better decoupling between the channels. You can connect only one input signal, and do not connect the second channel. The transmission ratio will not change. You can make different transmission factors for different channels (signals) by applying summation resistors of different values. In addition, you can use several channels.
If you need to amplify the total signal, then, in the case of an inverting adder and an additional inverting amplifier, you can set a smaller gain to one cascade and get a larger gain band.

 

I believe that the inverting combiner has a better decoupling between the channels. You can connect only one input signal, and do not connect the second channel. The transmission ratio will not change. You can make different transmission factors for different channels (signals) by applying summation resistors of different values. In addition, you can use several channels.
If you need to amplify the total signal, then, in the case of an inverting adder and an additional inverting amplifier, you can set a smaller gain to one cascade and get a larger gain band.

Thanks for your reply.
You mean the inverting summing amplifier is better? But then i need a additional inverting amplifier.
My problem is if i use a summing amplifier, i can't get a right sum of the two inputs. And i don't know why. Now i use OPA684 as adder.
Do i need a isolation between the inputs and the summing amplifier? Or which amplifier is better?

 

Can you describe your task in more detail? What are the input signals? What is needed at the output? What is the supply voltage?

 

Can you describe your task in more detail? What are the input signals? What is needed at the output? What is the supply voltage?

My task is to add a triangular waveform as dither signal to a voltage input and in this way i can make the performance of ADC better.
Now the two inputs are: one dither signal +-0.6mV with 2MHz, the other one normal input such as variable voltage between +-5V.
And now i try to use OPA684 as the summing amplifier, but now i can't get a right sum of the two inputs. I don't know why.
Can you help me to solve it? Thanks.

where VMID: 2.5V, VCC:5V, TRI: Dither, Adder: Output of sum.

 

I have a big suspicion that you did not set the opamp mode correctly. On a non-inverting input (for unipolar power), half the power is usually supplied (for example, from a resistive voltage divider).

 

Move your non-inverting input (+) to Vmid, then it should work correctly.

 

I have a big suspicion that you did not set the opamp mode correctly. On a non-inverting input (for unipolar power), half the power is usually supplied (for example, from a resistive voltage divider).

Yes, i think so.
But i don't know how to set the opamp of OPA 684 correctly.

 

Move your non-inverting input (+) to Vmid, then it should work correctly.

You mean like this?

But the sum result is not correct yet.

I don't know why.

 

I do not see the adder. Or do you use half the power as the source?

 

I do not see the adder. Or do you use half the power as the source?

The opamp OPA684 is the adder. Now i use VCC = 5V as the supply. But it works not well.
And i don't know how to correct this circuit.

 

Note the small capacitance of the capacitors. These containers remove the harmful effect of teeth on the vertices of the triangles.

 

Note the small capacitance of the capacitors. These containers remove the harmful effect of teeth on the vertices of the triangles.
View attachment 136086

Hi Bordodynov,
thank you so much for your suggestion.
But now my biggest problem as #35 said, the result after the summing amplifier is not correct. I have tried many times, but i still don't know how to fix it. Could you help me about that?

Best regards,
Jackie

 

JackieChan.
The operating amplifier you use is not fully compensated. This can cause problems. In my circuit, without small capacitors, the transient response is poor. I'm not sure that you can add picofarad capacitors to your opamp.

 

JackieChan.
The operating amplifier you use is not fully compensated. This can cause problems. In my circuit, without small capacitors, the transient response is poor. I'm not sure that you can add picofarad capacitors to your opamp.

Hi Bordodynov,
i want to try your circuit in LTspice. But i don't know so much about this software and i have met this problem. Do you know why?

Before i used TINA TI, but it is not so common.