Electronic – op amp rail to rail voltage follower latching up


I have selected a TS922 op amp to use in a voltage follower configuration. The specs for the TS922 in summary are:

  • Rail-to-rail input and output
  • Low noise: 9 nV/√Hz
  • Low distortion
  • High output current: 80 mA (able to drive 32 Ω loads)
  • High-speed: 4 MHz, 1 V/μs
  • Operating from 2.7 to 12 V
  • Low input offset voltage: 900 μV max.
  • Latch-up immunity

The voltage follower configuration is

voltage follower

Supply voltage is 5 V.

To test I am using a simple 10k pot arrangement (range v supply to gnd) to apply a voltage to the input (pin 3) with a voltmeter measuring the output on pin 1.

My issue is that the TS922 output latches up to 4.7 V if the input voltage rises above 3.6 V. If the input voltage is less than 3.6 V the TS922 behaves as a voltage follower.

To try to identify why this is happening I have tried:

  • placing a light load (2k resistor) from pin 1 to ground – no effect
  • grounding the inputs to the other dual amp in case there was some interaction – no effect.

Is there something I am missing?? Otherwise I suspect the markings on this device don't match what is inside.

I would ALSO appreciate any advice on other op amps which are close to the TS922. The most important specs are:
– single supply 5 V.
– rail to rail input/output.
– high output current capability > 50 mA.

I know I can search for devices – but I am hoping someone has experience with a device that works well.

Thanks in advance

Best Answer

A couple of days ago I tested some opamp in voltage follower configuration. All op-amps were bought from a reputable vendor (TME).

enter image description here

The input signal was coming from the function generator, \$F = 1\textrm{kHz}\$ and the signal amplitude was \$10V\$ peak to peak.

And all Opamp's was also supplied from symmetrical supply \$+/-5V\$.

Hence we can easily examine the allowed input common mode voltage range.

The first one was \$\textrm{LM}358\$ from ST.

enter image description here

As you can clipping accrue for \$Vin > (Vcc-1.4V)\$

And no sign of a famous crossover distortion due to lack of load resistance.

But reducing the input signal amplitude and adding a load resistance reveals the crossover distortion.

enter image description here

The second opamp was \$\textrm{TS922}\$ from ST

enter image description here

As you can see \$Vin = Vout\$ as expected form rail-to-rail input and output opamp.

No sign of "latching-up".

Hence, if your setup is correct I vote for a counterfeit part from China. Not a big surprise either.

Next one was \$\textrm{TL}072\$ from Ti.

enter image description here

As you can see we got a clipping for \$Vin > (Vcc-0.6V)\$ or so. And when the input signal approaches \$V_{EE} - 1.4V\$ phase inversion phenomenon manifests itself.

And finally the \$\textrm{NE}5532\$ from Philips.

enter image description here