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I found a couple of TI op-amps (opa688 or opa689) that should do it except the opa688 is obsolete and the replacement is SOIC (i need DIP) and the opa689 datasheet says it is only stable @ Gains 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I found a couple of TI op-amps (opa688 or opa689) that should do it except the opa688 is obsolete and the replacement is SOIC (i need DIP) and the opa689 datasheet says it is only stable @ Gains 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





RE: Voltage-Limiting Op-Amp
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.
RE: Voltage-Limiting Op-Amp
RE: Voltage-Limiting Op-Amp
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
RE: Voltage-Limiting Op-Amp
RE: Voltage-Limiting Op-Amp
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?
RE: Voltage-Limiting Op-Amp
RE: Voltage-Limiting Op-Amp
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RE: Voltage-Limiting Op-Amp