Looking for a CMOS drop-in for an LM324, good for +/-15V - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: December 16, 2016, 08:38:39 am
content: Hey there,Does anyone have a favourite CMOS drop-in for an LM324 which can handle running from +/-15V dual supplies?My usual go-tos, the TLC27L4 and LMC6484, are only good to 15V single-supplyCheers,

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2016, 09:43:11 am
content: With a +-15 V supply, there usually is no need for single supply anymore. CMOS OPs, especially the more modern ones are usually only for lower voltage. There are a few higher higher voltage CMOS, like the old CA3240 (dual). If you don't need CMOS, but can use a JFET based version, the TL064 might be an option, though not single supply.

author: nfmax
author_rank: Super Contributor
timestamp: December 16, 2016, 09:54:02 am
content: Why does it have to be CMOS? Just power consumption?

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2016, 01:08:33 pm
content: Why are you insisting on CMOS?CMOS is an inherently low voltage IC structure, due to the thinness of the gate oxide, which can't take much more than 20V before breaking down. Even the old CA3240 has a maximum differential voltage of 8V, even though the supply voltage can be much higher.JFET op-amps can have very low bias currents and high input impedances, so you could go for the OPA4140.http://www.ti.com/lit/ds/symlink/opa4140.pdf

author: Brutte
author_rank: Frequent Contributor
timestamp: December 16, 2016, 02:00:32 pm
content: That depedns on "drop-in" definition. If that is the price - forget it, you won't find anything cheaper than LM324.The pinout is kinda standard for quad opamps.fet high input impedance is the main advantage if needed (much lower bias current than 100nA from bipolar) but the offset voltage is higher within same price range. Perhaps just tweak the circuitry for matched impedances of inverting and non-inverting loads and then biassing current does not matter.

author: rob77
author_rank: Super Contributor
timestamp: December 16, 2016, 02:10:55 pm
content: don't forget that LM324 has the negative rail included in the common mode input voltage range, and for example the suggested JFET TL064 does not ! but on other hand the TL064 has the positive rail in it's common mode voltage input range. so basically from input point of view they are completely different beasts.

author: philpem
author_rank: Frequent Contributor
timestamp: December 22, 2016, 01:51:20 pm
content: Quote from: nfmax on December 16, 2016, 09:54:02 amEMC.Having some issues with bipolar junctions acting as rectifiers (actually, they're working as bloody brilliant crystal radios!) when exposed to ambient RF fields and making the circuit misbehave.Due to mass limitations, ferrites and metal shielding are on the "nope" list, and RC filters aren't quite cutting the mustard.PCB layout is actually pretty reasonable (2-layer with ground plane both sides) but I can't touch it anyway as the CAD files are long gone.I can guess why it uses 15V VCC -- the control input is 0-10V, "status" output is the same. Add some overhead for the opamps, you're at 12V, add so,me safety margin and that's 15V. Something else needed a negative supply, an ICL7660 gives you negative-15V-ish, which is "just easy".

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2016, 03:44:19 pm
content: Quote from: philpem on December 22, 2016, 01:51:20 pmThat is funny: 2 good ground planes makes this a single capacitor. Chances are you ended up with two sides cut to pieces that were meant to be ground, but now work as antenna.So the circuit is actually using +-15 V - thus single supply is very likely not needed. There are very few CMOS chips for higher voltage (the only ones I know are the old CA3240 (dual) and expensive AZ OPs for high voltage). I would give the TL064 / TL074 a chance and add a few more caps and maybe a few GND links / bridges to patch a broken GND plane. Analog has a few OPs with integrated RF filtering.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: December 22, 2016, 03:55:18 pm
content: Quote from: philpem on December 16, 2016, 08:38:39 amIf SOIC package is OK and the price is not very critical, try OPA4140 . It has JFET inputs, EMC protected and works from the negative rail on the input.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2016, 04:36:07 pm
content: Quote from: philpem on December 22, 2016, 01:51:20 pmDo you believe a MOSFET amplifier will be any better? Don't forget the ESD protection diodes connected between each input and the power rails.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2016, 04:42:55 pm
content: Ti seem to have some newer high voltage CMOS. The TLV4171 should reasonably fit, if you can get the right case. It might be a little fast however, but includes EMI filters.