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TL082: TL082

Part Number: TL082
Other Parts Discussed in Thread: LMV358, TLV272, LM358, LMP7702, TLV9102, OPA2990

Hi, i have an existing design that uses a TL082 as a volttage follower with high input impedance voltage divider in the range of 10m ohms and 1m ohm. The op amp is supplied with +5vdc/-5vdc, decoupled of course. Voltage is sent to a ADC (pic processor)

Usage is to read a battery voltage in a complex system.

it worked perfect for many years but now it needs to be upgraded for improved performance.

The only problem is the inability of the TL082 to not go rail to rail output.

i went over several test with other op-amps i have on hands and i dont have very good results. Main problem with thoses "better" op-amps that goes RR is in/out difference in the range of 1mv to 20mv which is not acceptable. The TL082 gives me expected precision but no rail to rail.

So my question is, what op-amp would be an immediate upgrade/replacement of the TL082 to achieve rail to rail output ?

how about a TLV272 ? I tested the LMV358 which seemed a good candidate but it was not at all.

thanks for any help.

Regards, Pat Allen

  • Patrick,

    TLV272 should do well. It can run from the +/-5V supply and has a rail to rail output. Input current is also low enough for your voltage divider resistor choice.

    LMV358 has a 5V maximum supply and +/-5V is effectively 10V. Input current is also much larger. 

  • i will try the TLV and will let you know.

    what are the key parameters i should look for to have same performance as the TL082 ?

    low input offest/current ?

    offset voltage ?

    thanks.

  • During my test qualification i fryed several LMV358 and i dont know why. same board, same parameters as for the TL082. +/- 5v rail.

    i tossed it away anyway,

  • Hi Patrick,

    can you please explain what you mean by the following sentence:

    "Main problem with thoses "better" op-amps that goes RR is in/out difference in the range of 1mv to 20mv which is not acceptable."

    Do you mean "offset voltage"?

    Well, for a replacement of TL082 you should look out for low "input offset voltage" and low "input bias current".

    Kai

  •  Kai, my problem is that there is a large difference in the voltage input versus the output of the voltage follower. My understanding of the leakage would have make the voltage lower, but in fact with the test results the output is higher than the input in the range of 10-60mv. Feels like the follower amplifies something Or there is a voltage that back flows somewhere in my voltage divider (with the LMV358 at least). The TL082 works at it best but no RR.

    test-adc.sch

  • Patrick,

    LMV358 is a 5V part and the power supply ESD clamp (inside LMV358) will draw a lot of current with 10V applied. The op amp sees +/-5V as being 10V.

    Is R1 is 10M ohms (where M is million , not m = mili , 1/1000)?

    What is the part number of the op amp that reads higher output? If it is LMV358, LM358 or any other op amp with a bipolar PNP input stage then output will be VIN + Vio + Iib * R1,  where Vio is input offset error, Iib is many nanoamperes of input bias current (use absolute value in the formula).

    Leakage could be another factor.

  • R1 is 10 megoohms, yes. the voltage divider is not shown so the voltage follower is 1:1 in this case.

    actual failed parts was LMV358aidr. i believe i read the p/n wrong.

    previous other part was LMV358 IDT from ST. sorry i realise now it is not a TI part. my fault

    will test the TLV soon and will let you know.

  • Hi Patrick,

    the LMV358 can only withstand +5V supply voltage (or +/-2.5V). +10V supply voltage (or +/-5V) is too much and will destroy the LMV358.

    Can you show a schematic with your actual voltage divider? And what are the value of C1 and C2?

    Kai

  • Morning Patrick, well - 

    1. The TL082 is a dual, assume you would like to continue with that 

    2. Your ADC is probably 0V ot 5V supply so your +/-5 op amp supply is ok if your input does not force below ground and will support a true swing to gnd operation

    3. The RR input stages usually have a crossover that passes control with an offset shift some amount below the positive supply 

    Anyway, look at the LMP7702 - RRIO, very low offset with only a 50uV shift about 2V below the positive supply and can support +/-5V. 

    200fA input bias current will not create issues even with 10Mohm source R - do not add a matching feedback R as offset current issues are not valid in this type of part. 

    http://www.ti.com/lit/ds/snosai9h/snosai9h.pdf

  • i use the 2nd opamp as an inverting 1:1 amp to another channel of the adc, to read positive or negative voltage from the input divider. every software loop i check both channels, if one shows voltage then i stay on this channel, i mute the other. it worked good in a actual circuit currently manufactured and vastly in use so no worries there.  i just need to have rail to rail operation now.

    In the real circuit there are divider resitors to a 4051 so i can change the divider ratio, 2:1, 20:1, 100:1,  but right now the focus is put on the "no divider" exactly as shown in my schematic above, so the follower act as a 1:1 in this case, so 0-5v positive input only. Works perfect with the TL082 but cant reach full scalle. Other op-amps inserted an offset to the voltage in the range of 10-20mv. lets say i put 1v at the input of the circuit, with anything but a TL082 i get 1.02v more or less at the output.

    Input capacitor C1 is a 1nf, C2 is a 0,1uf

    my mistake for the LMV358 it was wrong from the start.

    the LMP7702 seems a good candidate but quite expensive imho.

  • Well if you have volume, ask for a quote and tell am what you want to pay. 

  • volume wont fix the issue and may potentialy add 2-3$ more to the design, we would need to buy 10k+ pieces to achieve that which will never happen in this case.

    500-1000 pieces wont cut the price much to justify to switch from a 0,50$ piece to a 5$ one.

  • Yes, that does seem high - I had not looked, you would think there might be lower cost equivalents in the wider universe. 

  • Hi Patrick,

    decrease R1 for a test. Is the error becoming the smaller the more you decrease R1?

    Kai

  • it decrease the insertion error a litle bit but not much with the 358.

    i have to stick with the 10megohm resistor because it will affect all the divider network and i need to stick with a 10meg input resistance madatory for such kind of equipement.

  • Hi Patrick,

    what voltage drop do you measure across R1?

    What happens when you increase C1 to 100nF?

    Kai

  • i cannot do static measure of the voltage drop across R1 the value is too high my multimeter affect the reading (10 meg input too, Fluke 88I-3 ) actually when i try to do so i inject noise with my probes and i see output of the follower affected a lot.

    increasing C1 has no effect

  • actually i measure 0mv with the TL082 and about 10mv with the LMV358 (ST part)

    i have a fluke 8810A with much higher input impedance i will try with this one

  • Hi Patrick,

    the LMV358 (ST part) has an input bias current of 10nA at room temperature. This current will cause a voltage drop of 10nA x 10M = 0.1V across the 10M resistor. This adds to your input voltage and appears as additional offset voltage at the output. So, the input voltage and the output voltage are no longer the same.

    The TL082, on the other hand, has an input bias current of only 30pA. This current will cause a voltage drop of only 30pA x 10M = 0.0003V. So, when using the TL082 the input voltage and output voltage are nearly the same.

    By the way, I hope you decrease the supply voltage when working with the LMV358?

    Kai

  • First, the LMV358 (ST part):

    Then, the TL082:

    Kai

  • hi

    i completely tossed away the 358 for now

    i ordered other op amps with low input currents and will work soon on them

    thanks for the explications

    regards,

    Pat

  • Pat,

    You are on the right track. Any op amp with less than 1nA of bias current will only introduce an error equal to IIB * R. For example 1nA * 10Mohm = 1mV

  • Hi Patrick,

    I would get a closer look at the TLV9102 and OPA2990.

    Kai