Clamping op-amp inout < 0.3V - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: May 21, 2021, 09:07:54 am
content: Device = LMC6484Voltage at Input/Output Pin (V+) + 0.3V, (V−) − 0.3V.Current at Input Pin ±5 mAThe application sees a sine wave input; this is clamped "towards 0V" during reverse polarity. The forward impedance of the input network is about 4k. Max reverse current (when a clamping diode steps in) is only about 5mA. I want essentially all reverse current to go into the diode of course.I cannot see on the DS whether simply respecting "Current at Input Pin" guarantees I have no worry about "Voltage at Input", so I assume not.As discussed in other threads here, low Vf implies high leakage.Some op-amps (The ADA4177 for example) use depletion MOSFETs in series. I am unsure how easy this is to use with external discrete devices, especially thinking about the spread in reverse Vgs required.Also: Suppose I have a resistor in series with the input. I have no way to guarantee the resistance is large enough that I am within DS spec when using a standard Schottly clamping to perhaps -0.4V.Is there a simple solution, not involving using a -ve supply rail or hanging the reverse clamp diode from a given small +ve potential?Finally, should any input pin series (protection) resistor go after the clamp diode or before it?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2021, 11:04:39 am
content: Depletion FETs can be used to limit input current. The main incentive is to get less resistance and thus less resistor noise. The spread in threshold gives a spread in the current limit - usually not a big deal as the scattering is not that large with depl. MOSFETs as with JFETs. With a relatively high noise to start with a simple resistor may be sufficient.For high voltage spike one may want both, like 33 K + depl. Fet(D-S) 1 K (set current of FETs) dep. FET (S-D) .As usualy the limit is -0.3 V or a current limit to 5 mA. So it is not absolutely needed to limit the voltage to 0.3 V, if the current is limited. The information is a little hidden in a footnote:"Limiting input pin current is only necessary for input voltages that exceed absolute maximum input voltage ratings"The simple solution is us diode clamping to some 0.5-1 V and than a resistor towards the OP. So a resistor ( e.g. 1 K) between the OP and the clamp and a resistor (optional with dep. fets) in front.

author: amishasingh
author_rank: Contributor
timestamp: May 21, 2021, 11:17:05 am
content: Can you attach the schematic if you can.

author: StillTrying
author_rank: Super Contributor
timestamp: May 21, 2021, 12:47:14 pm
content: Quote from: 741 on May 21, 2021, 09:07:54 amAgree with Kleinstein, it does, limit the input current to <5mA on the peaks of the sine wave and you don't have to worry about the +/0.3V.Page 24 on the DS.https://www.ti.com/lit/ds/symlink/lmc6484.pdfYou'll probably need the input sine to be DC coupled, because an AC coupling cap would charge.

author: 741
author_rank: Frequent Contributor
timestamp: May 21, 2021, 01:21:23 pm
content: OK Thanks. I think this is what I already have.About "Note 12: Limiting input pin current is only necessary for input voltages that exceed absolute maximum input voltage ratings"I see that as only "perhaps implying" the corollary<Limiting input pin VOLTAGE is only necessary for input CURRENTS that exceed absolute maximum input CURRENT ratings>I once put this question to Microchip Support regarding PIC input pins:I asked whether (provided I respect current limit), I could increase the voltage limit?They replied "We do not recommend exceeding the maximum input supply voltage rating"

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2021, 02:12:03 pm
content: Quote from: 741 on May 21, 2021, 01:21:23 pmIt makes perfect sense. If the input is an ideal voltage source, i.e. a very low impedance, then it should not exceed the voltage rating of the input pin. If it's current limited, the current should be below the input's maximum current rating.The reason for this is the inputs are connected to the power supply rails, via diodes, which have a maximum current rating. If the input voltage is too low for the diodes to conduct appreciable current, then no current limiting is required. If the input voltage is high enough to bias the diodes into conduction, the current needs to be limited to a safe level.

author: 741
author_rank: Frequent Contributor
timestamp: May 21, 2021, 02:26:29 pm
content: Okay - many thanks all - I missed the note about page 24 which does give explicit permission to do this

author: StillTrying
author_rank: Super Contributor
timestamp: May 21, 2021, 03:04:33 pm
content: If you are using an op amp as a high impedance peak detector there's another 0.3V problem you often come across.https://www.eevblog.com/forum/projects/opa2388-peak-hold-detector-circuit-affected-by-opamp-input-leakage-current/

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2021, 06:49:48 pm
content: With a quad / dual OP, there is a chance that current through the protection diodes because of exceeding the supply range may lead to some surprising effect on othe pins (e.g. leakage to other pins). The protection diodes may be parasitic diodes to the substrate and this can cause odd things. It depends on the details of the OPs and many OPs may not be effected of this. With µCs and CMOS switches this a known hidden, possible trap: one ADC input seeing some current to the protection diodes and other ADC input showing extra leakage or wrong readings.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2021, 07:02:31 am
content: The schottky diode will do fine.Tim

author: 741
author_rank: Frequent Contributor
timestamp: May 22, 2021, 12:21:01 pm
content: Out of interest - is there any advantage in placing the input 1k before or after the Schottky?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2021, 12:48:18 pm
content: Beats me. What's it doing anyway?And what's supply voltage?Evidently not more than 16V, which makes that 24V TVS a rather odd choice. Why not use a pair of clamp diodes?Also the P6KE is pretty big for a resistor divider like that, a cheaper zener or small TVS will do. It can only see any substantial current due to high voltage breakdown of the outer resistor, as from ESD and surge.Tim

author: 741
author_rank: Frequent Contributor
timestamp: May 22, 2021, 01:32:32 pm
content: The TVS shown is 8v2 (not 24V).Supply is 9V.Thanks for prompting me to review those OTT TVS - I had some lying around...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2021, 04:11:05 pm
content: Oh, 8V2A, misread that as V24 at a glance.Tim