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Multisim, dac simulation

Hallo!

I'm trying to visualize the output signal from ADC ADS1252. The presented circuit uses a repeater for voltage reference. For some reason, the output signal is cut off at the top, if i apply the reference voltage through a repeater. If i remove it and apply voltage directly, the problem disappears.

And one more problem: after a few seconds of simulation an error occurs (see the log file).

Thank you!

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Hello viktorro,

 

I don't understand this part of the schematic. Could you please explain why did you connect noninverting input of the first op-amp to inverting of second one?

 

Capture.JPG

 

 

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This part looks like voltage follower but as you connected it to the positive terminal of the first amplifier it puts additional voltage reference in the noninversing input. I have removed it from the project and now it working. The modified file is attached.

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Thank you for answer! I want to create the simulation for schema from data sheet for ADC ADS1252 (see attached figure).

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I think on the first schematic the op-amps should be sourced from the bipolar power supply.

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Hallo! viktorro and Hello VA.KI,

 

First, not a major problem, based on the image (1.JPG) posted by viktorro, the circuit is an ADC not a DAC. Currently, I am only using a low-end mobile phone. It's inconvenient for me to open the product page or datasheet of ADS1252 so you might want to do the verification of the function this device performs.

 

I am only using Multisim Live Free Subscription so I cannot open the attached multisim (.ms14) file. I will just refer to the image posted by VA.KI (Capture.JPG).

 

 

@VA.KI wrote:

This part looks like voltage follower but as you connected it to the positive terminal of the first amplifier it puts additional voltage reference in the noninversing input. I have removed it from the project and now it working.

Initially, VA.KI got confused with how the resistor and op amp U4A were wired. Just when you realized that it "puts additional voltage reference" you removed this voltage follower. The simulation may have proceeded with no error or warning but it doesn't mean the circuit has already worked properly.

 

The first op amp (U4A) is a reference buffer. Its input is the reference voltage; its output drives the reference input (Vref) and the negative side of analog signal input (-IN) of ADS1252 while also providing a DC input voltage to the second op amp (U2A). The circuit formed by U2A is simply an active level shifter, the DC voltage from U4A and the resistor network determines how the input signal should be shifted. Deleting the buffer U4A may not have significant impact to proper operation of the circuit because the voltage reference is an ideal source.

 


@VA.KI wrote:

I think on the first schematic the op-amps should be sourced from the bipolar power supply.


This is not correct, this will defeat the purpose of employing op amp U2A. The purpose of using this active level shifter is to allow the ADC (ADS1252) to process bipolar input signals while only using a unipolar power supply for the entire circuit. You can get a hint of what the circuit is supposed to do by reading the caption for FIGURE 1 in "1.JPG".

 

The circuit should work properly if you used 2.5 V instead of 5 V for V7, the reference voltage. Furthermore, the signal generator should be configured so that its output is within ±5V. This will keep the signal inside the proper working range of the level shifter.

 

Open "1.JPG" and carefully note that the reference input is 2.5 V. Compare this with V7 in "Capture.JPG" which has a value of 5 V. Note also from the table in "1.JPG" that with the resistances used in "Capture.JPG", the circuit is for accommodating ±5 V input.

 

 

Best regards,

G. Goodwin

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