Help with adding more voltage offset to an opamp level shifter circuit? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 29, 2018, 04:16:08 pm
content: I have built this circuit to level-shift a composite video signal to have no portion of the output waveform below 0V, and it works perfectly as per the schematic:Here's the working circuit with the usual rev 1.0 bodgesNow, I need to add an extra +2v offset for it to be compatible with the CRT I am hacking (To inject the signal in after the RF circuitry I am removing)..I will increase the supply voltage to 5v to allow for the higher output voltage, but can anyone help me calculate the values of the resistors R1 and R2 etc? I can lay out a PCB and stuff, but my maths game is a little weak......The circuit is detailed in this datasheet, page 24.http://www.ti.com/lit/ds/symlink/lmh6601.pdfThanks!

author: slugrustle
author_rank: Frequent Contributor
timestamp: April 29, 2018, 05:27:49 pm
content: QuoteThe output of the reference circuit is 0V - 2V. Do you want your output to be 2V - 4V, or do you want it to be 0V - 4V?

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 30, 2018, 12:38:59 am
content: Ah, sorry I wasn't clear.I need the signal to be from 2v to 4v.

author: slugrustle
author_rank: Frequent Contributor
timestamp: May 02, 2018, 04:02:13 am
content: Thanks, and thank you for linking the datasheet.I'm looking at Figure 53 on page 24. The goal is to select R1 and R2 such that the voltage at the U1 non-inverting input has a lowest value one half as large as its highest value when a -0.3V to 0.75V signal is present at VIDEO_IN. This way, I can get 2V - 4V out by multiplying by 4 / (highest value at U1+).I wrote a brute force program to iterate through all E24 and E96 resistor values to meet that criterion, and used a voltage divider calculator to find the gain resistors. The end result is the attached schematic and simulation. The LMH6601 model comes fromhttp://www.ti.com/lit/zip/snom160. I imported it into LTspice followinghttp://www.analog.com/en/technical-articles/ltspice-simple-steps-to-import-third-party-models.html. I attached the .asy file inside LMH6601.zipI did not check thermals or power. Outputting 2V-4V into a 150ohm load could overheat your chip.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 05, 2018, 07:30:08 am
content: Hi,Thanks for that, but it looks like your schematic is different to the one int he datasheet.. Seems like you added R1, but missed out R3. I think this might alter the values a bit?

author: slugrustle
author_rank: Frequent Contributor
timestamp: May 05, 2018, 02:18:47 pm
content: Ah, R1 is just to model source impedance. It doesn't end up in the thing you build. I attached a schematic that should be more clear.R3 in the datasheet provides a negative shift to ensure the low range of the output is at 0V. I used a different scheme to get 2V-4V on the output: I adjusted R1 and R2 so that the opamp +input voltage ranges from 0.8267V to 1.653V and set the gain of the opamp to 4V / 1.653V. Because 0.8267V is one half of 1.653V, you get 2V-4V on the output without needing R3 to add a negative shift.The source voltage in the simulation is -0.6V to 1.5V, so with the extra 75 ohm source impedance resistor, this simulates -0.3V to 0.75V on VIDEO IN. The simulator gave 2V-4V on the output.I will post the circuit analysis and R1/R2 resistor finder program later today.

author: slugrustle
author_rank: Frequent Contributor
timestamp: May 06, 2018, 03:17:21 am
content: I've attached the circuit analysis that I did for R1 and R2, along with the JavaScript calculator that I wrote to find optimal values for R1 and R2. The calculator is in a zip file because we can't attach html files. Just unzip it and open it with your browser; all it does is print results to the screen.I used the Voltage Divider Calculator athttps://slugrustle.github.io/to find the opamp gain resistors. I just entered "1.6534578 / 4" (without quotes) in the "Target Divider Ratio" box and hit "Solve".1.6534578V is the voltage at LMH6601 non-inverting input when VIDEO IN is 0.75V for R1 = 15.4k and R2 = 4.12k.This was a fun problem. It took me a while to remember how to do node-based circuit analysis.