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Yet Another Jellybean Opamp

Hi,

I'm looking for a single supply jellybean opamp to meet the following requirements:

V supply = 5.0V Vin (e.g. Vicm) 0V to 4V. Iin +/- 100pA max. (prefer < 10pA)

Other parameters are "normal" e.g. slow, noisy, 1mA drive, 0-70 degree C, up to $2 for a dual, etc..

Here's the hard part: I want the Iin spec to apply over the Vin range

*even when the supply voltage is zero*. This requires the absence of ESD protection diodes between the inputs and the positive rail. Newer opamps almost always have these diodes.

There are plenty of cheap older opamps that can come close to meeting my requirement, e.g. the ubiquitous LM358A and its siblings would be fine except that Vicm only gets to Vsupply - 1.5V or so.

Microchip have some interesting parts, e.g. MCP6L02 and MCP6002 are about US$0.25 in my quantities. They explicitly state in the datasheet that they don't have diodes from the inputs to the positive supply rail (good) but the absolute maximum ratings say that "Analog Inputs (V IN+, V IN?) VSS ? 1.0V to VDD + 1.0V", which isn't so good.

Does anyone have experience with those Microchip parts, and know whether the abs max ratings can be exceeded without problem? Normally I would never do that, but I suspect in this case that the datasheet might be overly conservative.

Any other opamp suggestions?

If I can't find an opamp that meets these requirements I can use other circuit topologies that aren't as elegant as the one I have in mind.

Thanks, Allan

Reply to
Allan Herriman
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Something like this is pretty compact, I'm sure you'll find a problem with it, "max" leakage spec sucks e.g.:

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Reply to
bloggs.fredbloggs.fred

If you want near zero input bias current even when powered down then what about a series mosfet to isolate the op-amp input from your source when the circuit is unpowered - then you have a much wider choice of op-amp?

piglet

Reply to
piglet

Thanks. That part has a recommended minimum supply voltage of 16V though, so it doesn't meet the +5V requirement.

Regards, Allan

Reply to
Allan Herriman

Thanks piglet. That was one of the "other circuit topologies" that I alluded to in my OP. I use a Diodes Inc. FET with low Vth and low cost (can't remember the part number right now) that would be suitable in this application.

Regards, Allan

Reply to
Allan Herriman

Maybe a diode in the positive supply line.

Hul

Allan Herriman wrote:

Reply to
Hul Tytus

What about an input relay, coil powered from your 5.0V supply? DPST-NO would lift one input from two op amps, or both inputs of a single op amp, and some of 'em don't eat your whole budget.

Reply to
whit3rd

Whilst that would protect the opamp input, it wouldn't meet the 100pA Iin requirement, as the input would end up powering the opamp through the ESD protection diode. (Assuming I've interpreted your suggestion correctly.)

Thanks, Allan

Reply to
Allan Herriman

Thanks for the suggestion. That particular relay operates at 3.75V, so it would probably need to be driven by a brownout detector or similar.

I wouldn't use a relay for a "dry" switching application like this though.

Regards, Allan

Reply to
Allan Herriman

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It is used at 5V but it loses rail-to-rail performance. There's the MAX366 spec'd for LV but the channel starts going hi-z for Ain within 1.5V of rail . And all these parts are pricey, in the $3-4 dollar range, so not jellybea n. In the way of OAs there's the OP191 and later derivative generations, bu t pricey too.

Reply to
bloggs.fredbloggs.fred

I hadn't come across the OP191 before. They only specify the input current for voltages between the rails. Looking at the schematic (figure 61 in the OP191 datasheet) it seems that it actually does have clamp diodes to the positive supply and also back- to-back diodes between the inverting and noninverting inputs. There are

5k series resistors to limit the input current. Unfortunately this won't meet my Iin requirement.

The FET I was thinking about earlier (in my reply to Piglet's post) was the DMG1012UW. It has a threshold voltage of 0.5 to 1V @ 250uA, meaning I could use it to isolate the opamp input and still meet the 4V input with 5V supply requirement. I suspect that would work better than the Maxim parts you mentioned for voltages close to the rail.

The leakage current is ok if I only look at the typical parameters. They don't specify a max IDSS at anything other than 25 degree C (100nA). The typical curve looks good though (about 7nA at 85 degree C and 4V).

I use a lot of these FETs for driving things from 1.8V logic, so they're already on the board and only cost a few cents each.

Regards, Allan

Reply to
Allan Herriman

Allan - your interpretation's correct. My suggestion shouldn't work but, if I were in your shoes, I'd try it anyway.

Hul

Allan Herriman wrote:

Reply to
Hul Tytus

Mercury-wetted relays are nice for "dry" switching. They do have to mounted within 15 degrees of the vertical, which can be a problem, but they don't bounce on closing or opening - the mercury film provides mechanical damping.

--
Bill Sloman, Sydney
Reply to
bill.sloman

We use thousands of a tiny Fujitsu DPDT relay for low-level switching, like thermocouple inputs, and they work fine.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

I keep looking for an ROHS compliant Mercury-wetted relay but no luck so far ...

Allan

Reply to
Allan Herriman

I used to use Fujitsu relays when I worked for Fujitsu. Here though, it's the choice of a relay vs a US$0.10 FET that takes no board space, withstands any amount of vibration and will last forever in a corrosive atmosphere. [hyperbole]

Actually, I'm still hopeful of finding an opamp that does exactly what I want in one chip. Contemporary 74xx series CMOS logic gates have the sort of ESD protection I want (i.e. input voltages > Vcc without drawing a lot of current). Why can't an opamp? Are the processes so different? Does that sort of ESD protection have some bad effect on analog circuits (e.g. leakage current)?

Regards, Allan

Reply to
Allan Herriman
[snip]

Didn't your original post say even while unpowered? The only way to do that is with specially-designed ESD, such as is found in many bus transceivers. Off-hand I don't know of an OpAmp so constructed. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
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I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Keep in mind the FET has about 100pF of capacitance at 0V.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Hi Jim,

Yes, the requirement is |Iin| < 100pA under the conditions of

0 < Vin < 4 and 0 < Vsupply < 5V.

BTW, The Microchip MCP6002 family has that sort of ESD protection. Its abs max specs mean that I can't use it (Vin max is VDD + 1V) but I'm hoping there are others out there that I can use.

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Regards, Allan

Reply to
Allan Herriman

Thanks Win. In general that is a very good point, however in this application the voltage doesn't change very quickly and C dv/dt won't come close to 100pA.

Nice book, BTW.

Regards, Allan

Reply to
Allan Herriman

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