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
Yet Another Jellybean Opamp
Started by ●February 22, 2016
Reply by ●February 22, 20162016-02-22
On Monday, February 22, 2016 at 4:27:53 AM UTC-5, Allan Herriman wrote:> 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, > AllanSomething like this is pretty compact, I'm sure you'll find a problem with it, "max" leakage spec sucks e.g.: http://datasheets.maximintegrated.com/en/ds/MAX4505.pdf
Reply by ●February 22, 20162016-02-22
On 22/02/2016 09:27, Allan Herriman wrote:> 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 >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 by ●February 22, 20162016-02-22
On Mon, 22 Feb 2016 05:47:56 -0800, bloggs.fredbloggs.fred wrote:> On Monday, February 22, 2016 at 4:27:53 AM UTC-5, Allan Herriman wrote: >> 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 > > Something like this is pretty compact, I'm sure you'll find a problem > with it, "max" leakage spec sucks e.g.: > http://datasheets.maximintegrated.com/en/ds/MAX4505.pdfThanks. That part has a recommended minimum supply voltage of 16V though, so it doesn't meet the +5V requirement. Regards, Allan
Reply by ●February 22, 20162016-02-22
On Mon, 22 Feb 2016 14:10:33 +0000, piglet wrote:> On 22/02/2016 09:27, Allan Herriman wrote: >> 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 >> >> > 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? > > pigletThanks 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 by ●February 22, 20162016-02-22
Maybe a diode in the positive supply line. Hul Allan Herriman <allanherriman@hotmail.com> wrote:> 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 by ●February 22, 20162016-02-22
On Monday, February 22, 2016 at 1:27:53 AM UTC-8, Allan Herriman wrote:> 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*.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. <http://www.digikey.com/product-detail/en/EE2-5NU-L/399-11017-1-ND/4506469>
Reply by ●February 23, 20162016-02-23
On Mon, 22 Feb 2016 23:04:44 +0000, Hul Tytus wrote:> Maybe a diode in the positive supply line. > > HulWhilst 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 by ●February 23, 20162016-02-23
On Mon, 22 Feb 2016 15:41:07 -0800, whit3rd wrote:> On Monday, February 22, 2016 at 1:27:53 AM UTC-8, Allan Herriman wrote: > >> 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*. > > 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. > > <http://www.digikey.com/product-detail/en/EE2-5NU-L/399-11017-1-ND/4506469> 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 by ●February 23, 20162016-02-23
On Monday, February 22, 2016 at 4:29:37 PM UTC-5, Allan Herriman wrote:> On Mon, 22 Feb 2016 05:47:56 -0800, bloggs.fredbloggs.fred wrote: > > > On Monday, February 22, 2016 at 4:27:53 AM UTC-5, Allan Herriman wrote: > >> 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 > > > > Something like this is pretty compact, I'm sure you'll find a problem > > with it, "max" leakage spec sucks e.g.: > > http://datasheets.maximintegrated.com/en/ds/MAX4505.pdf > > > Thanks. That part has a recommended minimum supply voltage of 16V > though, so it doesn't meet the +5V requirement. > > Regards, > AllanIt 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 jellybean. In the way of OAs there's the OP191 and later derivative generations, but pricey too.
