Transform voltage level at OA output TLE 2141.

[nenea dani]

I have a follower with TLE2141 that receives at IN + a continuous voltage from AD8307. The power supply is +/- 5V. Without signal at output 6 of TLE 2141 we have 220mV. How do I make 100mV in this case keeping unit amplification? The voltage variation is 220-1330mV with signal and everything is fine. I have tried offset compensation as in the datasheet but at output the variation is only 5mV. No positive or negative voltage applied to the offset N1 or N2 lowers the 220mV output more than 2-3 mV. I tried with OP97FP and the result is pretty much the same. In this case with OP97 can only increase the voltage of 220mV to 1V.

 

[KlausST]

Hi,

I didn't get it.
Why not drawing a little picture?

Klaus

 

[d123]

Hi,

If I understand what you're describing, maybe make your supply V+ 5V and V- -5.12V. There's some TI part that creates a small negative voltage precisely for this kind of issue, I don't remember the part name, it appears in the Precision Labs Op Amps video tutorials somewhere or other.

I think offset null pins only function over a few mV.

 

[schmitt trigger]

Hi,

I didn't get it.
Why not drawing a little picture?

Klaus

Even a hand-drawn schematic will help.

Remember the old saying, a picture is worth a thousand words.

 

[nenea dani]

Dear friends thank you for your attention.
The schematic diagram is at http://www.dc4ku.darc.de/S_Meter.pdf



Bild (fig.) nr 4 . The original TL081 works just like in documentation. Why does not it work with TLE2141? Any voltage (+/-) or balance applied to pins 1 and 5 or in turn does not produce a major output difference, only 5mV. Is TL81 an old circuit and needs a vigorous compensation? I changed with 2141 because it seems more capable in terms of speed and thermal drift.

 

[FvM]

I don't believe that TLE2141 behaves better in this circuit than TL081. It's a bipolar OP with huge input current, not good choice. There are other low offset and high speed OPs that should be considered.

 

[Audioguru]

The input offset voltage null pins on a OPA2141 are completely different to an old TL081 and are used to null the maximum input offset voltage of only 1.7mV. You must use its inputs to cause a -120mV offset.

 

[nenea dani]

It seems to be definitive with the TL081. After several hours of operation it did not deviate from the output by more than 3-4mV at 100mV with 50ohm terminator on the input . Anyway there is some noise on the output. I went from a SI5351(10,7Mhz), a 3P pole BPF filter on 10Mhz hand made and then a 0-31db attenuator in the 1db step. I added then a 20db coaxial attenuator. Looks like I have an error of less than 1dbm. I still have RG316 patch cables and SMA adapters on the network. I put -2db for BPF and -1db for the route. We used tables that transform the rms voltage into dbm and a Rigol DS2072 set on 50ohm . It does not make FFT accurate but after the filter I've never seen any harmonics. I stop here and thank you all.