Voltage-Limiting Op-Amp

[Rick360]

I have a need for a circuit that has an input range of 0 - 5 VDC with an adjustable output voltage limit and adjustable(near unity) gain for an output between 0 and 5 VDC. Single rail 5V-10V Power Supply needed.

I found a couple of TI op-amps (opa688 or opa689) that should do it except the opa688 is obsolete and the replacement is SOIC (i need DIP) and the opa689 datasheet says it is only stable @ Gains > 4.

What happens when gain is less than 4? Will it oscillate? To use the opa689 should I scale the input by 4 then use gain of 4? (will increase noise)

Are there any other voltage limiting op-amps (i can't find any others on digi-key or google)

Thanks in advance

Rick

 

[VEBill]

If your power supply is single-rail 5v (meaning gnd and 5 volts), and the input and outputs are also expected to swing between 0 to 5 volts, then perhaps you're looking for an opamp with 'rail-to-rail' capability.

Oscillation is often caused by too much gain (waaay simplified...) - not too little gain.

Voltage limiters often involve diodes connected to a reference (adjustable if required) so that the input signal, through a resistor of course, cannot exceed the reference (plus the diode drop). If necessary, this can be done for both an upper and lower limit.

Your stated requirements do not appear to demand anything particularly unusual.

You should probably search for 'OpAmp' and 'Cookbook' to see a variety of OpAmp circuits.

 

[OperaHouse]

An open collector op amp like a LM339 can operate like an active zener to limit the voltage to a set level.

 

[Rick360]

Sorry for my limited op-amp knowledge.

VE1BLL wrote:
"Oscillation is often caused by too much gain (waaay simplified...) - not too little gain"

Why would the specs say it is stable @ Gains >4 and have a different part for gains <4?

VE1BLL wrote:
"Your stated requirements do not appear to demand anything particularly unusual."

The adjustable output limiting is the part I have been unable to figure out. How would the limiting circuit you mentioned work on an op-amp output?

OperaHouse wrote:
"An open collector op amp like a LM339 can operate like an active zener to limit the voltage to a set level."

How would this work?

Rick

 

[OperaHouse]

The LM339 requires a pull up resistor to operate like a standard op amp. To this amplifier stage output, you connect a second op amp output of this package. Connect the + input of this section to the output. Connect the - input to a reference voltage that is exactly the same as you want to limit the output.

 

[VEBill]

...stable for Gain > 4... ?

OPA689 Spec sheet: "This part is de-compensated (stable for gains >/= +4). This gives greater bandwidth, higher slew rate, and lower noise than the unity gain stable companion part OPA688." Refer to spec sheet for further details.

Interesting, I had never seen such a thing before.

Also on page 14 of the spec sheet: "OUTPUT LIMITERS - The output voltage is linearly dependent on the input(s) when it is between the limiter voltages VH (pin 8) and VL (pin 5). When the output tries to exceed VH or VL, the corresponding limiter buffer takes control of the output voltage and holds it at VH or VL."

I believe that this embedded feature makes the limiter function extremely easy. It seems that you just need two reference voltages corresponding to the desired limits.

I wonder if they provide application notes for that feature?

 

[VEBill]

For the minimum stable gain issue, just set the OpAmp gain to something greater than 4 and use a voltage divider (attenuator) on the input and/or output...

 

[VEBill]

Here is a good introduction to the basics of limiters:

http://www.tpub.com/content/neets/14181/css/14181_174.htm

You can browse around this link (Next, Back, Up) for more information.

 

[cbarn24050]

Hi, adgustable voltage clamping is often done with another op amp connected in the feedback loop. If you want to limit both upper and lower voltages then you need 2 amps. For example to clamp the upper value external op amp invinp to ref, noninvinp to main output, output to a diode anode, diode cathode to invinput of your main amp. To get stability at unity gain you need a compensated amp eg 741.