LM358: LM358P-Working Operation

/resized-image/__size/320x240/__key/communityserver-discussions-components-files/14/New-Board-at-U5.4.png --- New Board

/resized-image/__size/320x240/__key/communityserver-discussions-components-files/14/Old-Board--at-U5.4.png --- Old Board

Hi Srikanth, 

Could you clarify what the difference between the old board and new board configuration is? Where is the waveform taken in the schematic? 

What does the input at the amplifier look like as well? 

Regards,
Ashley

Hi Ashley kang,

● Could you clarify what the difference between the old board and new board configuration is?

- We have tried maintained the same design as Old for our new board. Same layout, same DAC, amplifier and counter part number.

● Where is the waveform taken in the schematic? 

- 

As we can't be able to check the current amp. So, we are referring to the voltage of the DAC output.

 ● What does the input at the amplifier look like as well? 

 --> Golden board

 --> New board

*Yellow is amplifier output.

*Violet is DAC output.

*Blue is MSB from the counter.

Regards,
Mirthunraju M

Hi Ashley,

Only MSB is shown in above capture but we are sure that all 8 bits are HIGH in both boards during the yellow color signal window gap.

It is taking more time to communicate here. This board is very urgent to our client. Is it possible to schedule Webex / Teams session for quick resolution?

Regards,

Srikanth Kacchu

What are the voltages at the inverting inputs (pins 2 and 6) of the amplifiers when the error happens?
(I suspect the voltages might be outside the valid common-mode range, i.e., above 13 V. In that case, replacing the LM358 with a rail-to-rail amplifier like the OPA2990 would help.)

Hi Clemens,

● What are the voltages at the inverting inputs (pins 2 and 6) of the amplifiers when the error happens?

- new board: at pin2, Vmin =-6.1V & Vmax= 92.4mV whereas, pin6 =0V

- old board: at pin2, Vmin =-5.77V & Vmax= 230.96mV whereas, pin6 =0V

/cfs-file/__key/communityserver-discussions-components-files/14/New-Board.png

/cfs-file/__key/communityserver-discussions-components-files/14/Old-Board.png

refer the below image from new board,

the PE signal is high from the Up/down counter that hold all input bits high (i.e 2^8) so, the amplifier out should be high at that time! But there toggling is happening in the new board compared to old board.

refer the below image for DAC out hold duration from the old board,

Regards,

Mirthunarju M

Hi Mirthunarju, 

Could you provide a zoomed in oscilloscope image of the good and bad where we can see the PE and op amp waveform in more detail? How does PE connect to the DAC or LM358? 

What is the load on Output [5] and where does it connect to in the schematic? 

Regards,
Ashley

Hi Ashley, 

● Could you provide a zoomed in oscilloscope image of the good and bad where we can see the PE and op amp waveform in more detail?

- Yes, please refer the below images,

--> old Board

--> new board

● How does PE connect to the DAC or LM358?

- PE is connected to the Counter CD4516BE- U1.1 & U2.1, this input going to impact on the counter output i.e., all the bits are high.

● What is the load on Output [5] and where does it connect to in the schematic? 

-  R11 going to act as load for Output [5] & Output [5] is connected to a connector.

Regards,

Mirthunraju M

Hi Ashley, 

We have simulated the LM358P,

where the input IG1 is current input max of 2mA from the DAC0808 datasheet where all the digital inputs are high.

--> old board case

 --> new board case

in old board case, the current input is constant & the amplifier out is hold(high) for certain duration.

in new board case, the current input is toggled from 0 to 2mA & the amplifier out is toggled as we discussed early.

where both DAC voltage output as similar from old and new board,

--> old board

--> new board

zoomed image are available in the previous chat.

Regards,

Mirthunraju M

Hello Mirthunraju,

It seems that the output is [Iin * -51] in parallel with [11.54V +  23.08k]. About 0.5mA to 102.5mA into a short to ground  So open load will quickly slam the op amp into output high (VOH, 13.5V) when even a small input is present.

I believe the T0 blanking clock is running during the time in question. So an ideal op amp would return output to around 11.54V. Therefore the new board that doesn't stick VOH seems like the better functioning system. 

Is there any advantage to sticking VOH? 

Is output really open load? What is the normal load? Do both boards work OK with normal load?

Hi Ron, 

● It seems that the output is [Iin * -51] in parallel with [11.54V +  23.08k].

- I can't able to understand this statement.

● About 0.5mA to 102.5mA into a short to ground So open load will quickly slam the op amp into output high (VOH, 13.5V) when even a small input is present.

- Yes, even if some uA, the output is high. According to the simulation.

● I believe the T0 blanking clock is running during the time in question. So, an ideal op amp would return output to around 11.54V. Therefore, the new board that doesn't stick VOH seems like the better functioning system. 

- Yes, T0 Clock is running during the time. But can't able understand why the +ve pulse and -ve pulse occurs in Vref+ and need to check VOH sinking. 

● Is output really open load? What is the normal load? Do both boards work OK with normal load?

- the current is directly produced from the DAC and straightly given to Amplifier. 

Regards,

Mirthunraju M

Mirthunraju,

The two op amps have a input current that comes from the DAC. The op amps amplify this negative input current by -51 thus providing a positive output current.  R11 & R15 (original schematic) provide a Thevenin equivalent of 11.54V + 23.08k ohms. So both circuits below do the same thing. 

Mirthunraju M said:

But can't able understand why the +ve pulse and -ve pulse occurs in Vref+ and need to check VOH sinking. 

The Reference current amplifier inside the DAC takes a finite amount of time to adjust to the sudden change in reference current.

I'm not sure why the T0 = low time had output near 7V ; that would be 3.9uA from DAC in or 200uA load on OUTPUT node (35k ohms).

Hi Ron,

why is the amplifier out is constant at highlighted period even the DAC output is toggling (460mV to -6V) ?

Is there any facility to arrange a call or meeting?

Regards,

Mirthunraju M 

Mirthunraju,

Mirthunraju M said:

DAC output is toggling (460mV to -6V

I see the -6V on purple, this is a large overdrive input. This is why yellow rise time are fast. 

The VIH, high level is much smaller, too small to discern here. 

When output reaches VOH level internal nodes continue to change voltage. Once the input request less than VOH, the over charged internal nodes need to come back to a linear range. The time between input change and output leaving VOH is called over load recovery time. With a large positive "DAC OUT" this will occur after a few microseconds. If the positive voltage is less than 120mV then the time becomes much longer. 

However the the length of VOH time here suggests that positive voltage is tiny (less than the offset error voltage) or not present at all. I believe not at all. Using a smaller vertical scale will make seeing DAC OUT high level more clearly. However keep in mind that oscilloscopes do have a small DC error.