Newb OpAmp Vout to 0v? - Page 1
Forum section: electronics / beginners

author: Audiofanman
author_rank: Contributor
timestamp: March 12, 2025, 12:32:05 am
content: Hello all!This is a kind of long one. I thought that a full explanation might help?Bread boarding my ltspice schematic has taught me that bread boarding will forever be a mustBEFORE spending time in kicad and ordering a few boards!!!ltspice circuit works perfectly, but in real life I am finding that Vout on the LT1097 just refuses to drop down to 0v..I tried a 10k pot with the wiper to V+ and the outer legs tied to Vos Trim (1 andas shown in the datasheet with no change.I have the LT1097 wired up as a voltage follower (I think lol)Vout fed back to the inverting input, The Non inverting input fed by the voltage I wanted to reproduce with less impedance on the Vout...+In is expected to be anywhere from 10mV to 6.5v (which has a high enough impedance to only supply a few uA's)I wanted to drive an analog meter movement with Vout based on +In.The LT1097 +In is being powered by a BK Precision 1670 Power Supply from 0-7V, andV+ being fed by a separate power supply consisting of a variac, 14-18V isolated output, B250D full wave bridge rectifier, filter cap, and lastly a LM7809 ...Dropping the +In to 0v (By disconnecting the BK completely) causes the Vout to jump to 0.663VLeaving the BK connecting but dialing the voltage down as far as it will go produces 0.440V on Vout but some how the BK display reads 0.4v(Strange, cause pulling a lead off that supply makes that 0.4 display drop to zero, like something is being fed backwards somehow..Once I have a little voltage dialed in, everything works as the ltspice sim, its only 0-1v on Vout thats the problem....I know this is long,Im gratefull for any advice on how to get Vout to track down to 0V

author: Whales
author_rank: Super Contributor
timestamp: March 12, 2025, 01:05:09 am
content: What voltages have you connected to the opamp's power input pins (VDD & VCC)? You only mention the positive voltage (LM7809->9V), what about the negative? Is it just GND?The LT1097's datasheet shows its output stage:The key bits controlling the output are Q33 and Q32. They are in "common collector" arrangement, which means this isnota rail-to-rail output amplifier. You cannot expect it to properly output signals within a volt or so of the power rails. Eg if you're powering it with 12V and GND (0V) then, as an approximation, don't expect it to be usefully able to output outside the range 1V-11V, or maybe 2-10V (depending on your load & driving speeds).This is not a rail-to-rail input amplifier either. The "Input Voltage Range" spec on the datasheet is only +-13.5V when you are feeding the device +-15V as power. Inputting a signal within a volt or so of one of the rails will lead to undefined behaviour.Nonetheless, have you tried adding a resistor between the output and GND? 1Kohm perhaps. That might fix the output swing problems (but it won't fix the input problems, which could still trip you up).There is a good chance your SPICE opamp model for this device only cares about "small signal" performance, ie stuff not near the voltage rails. It's a simplification that isn't always good.

author: dkfan9
author_rank: Newbie
timestamp: March 12, 2025, 01:28:52 am
content: Whales posted while I was typing a similar message, but I'll leave the attached LT design note which provides some estimated I/O ranges for 3V supply (and has some alternate possibilities if you need to replace).

author: PGPG
author_rank: Frequent Contributor
timestamp: March 12, 2025, 10:52:45 am
content: Quote from: Audiofanman on March 12, 2025, 12:32:05 amI'm not sure what BK in your description is, but if you disconnect OpAmp input you can't assume it gets 0V. Even input consumes zero, zero, nothing of current it is enough to get to other voltage than 0V.

author: Audiofanman
author_rank: Contributor
timestamp: March 12, 2025, 11:05:00 am
content: V+ is a rectified and filtered 9v, and yes the negative lead is just grounded to the negative rail from the rectifier.I understand about the headroom required, and since I need to monitor 0-6.5v chose a 9v regulator.I guess I'm going to try and find a rail to rail opamp.. good times lol..Doesn't appear to get many results with TO-220 packages..But now I'm getting myself into trouble lol...Since the trouble area is 0-1v and that will be fed from a 100/1 voltage divider, im thinking maybe for the 0-100vdc circuit I hope to monitor, i should change the divider circuit to a 10/1.. hmm..That circuit is never really going below 15v and shouldn't go above 80v, 1.5v to 8v is very workable..Thanks for the idea!!

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2025, 11:52:41 am
content: Why do you want a TO-220 package? They're only used for high power dissipation i.e. op-amps with higher output currents than 25mA and supply voltages above 24V or so. A completely different use case to the LT1097.If you need high power and precision, the usual way to do it is to add power transistors to a low power, precision op-amp.If you need a precision, single power supply, i.e. be able to work with its input and outputs close to the negative rail, then the LT1006 and LT1413 are good choices.https://www.analog.com/media/en/technical-documentation/data-sheets/LT1006.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/lt1413.pdfIf you're not bothered about precision and just want a cheap, single power supply op-amp, then there's the LM358.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 12, 2025, 05:21:43 pm
content: Quote from: Audiofanman on March 12, 2025, 11:05:00 amHeadroom is required not only at +, but at both sides.Quote from: Audiofanman on March 12, 2025, 11:05:00 amHaving stable voltage source (you have 9V) using resistors you are able to map 0..100V not only into voltage range starting from 0V (like 0V..1V) but to range not starting from 0 (like 1V..2V).

author: Whales
author_rank: Super Contributor
timestamp: March 12, 2025, 10:27:43 pm
content: Quote from: Zero999 on March 12, 2025, 11:52:41 amI'll +1 this.The LM358/LM324 (same thing) are the cheapest and most common opamps available today. Their performance is low in terms of bandwidth and offsets, but they might suit your use case perfectly.(1) Input down to negative rail is permitted.(2) Output can go near to negative rail if you use a pull-down resistor on the output.(3) Tolerates a wide voltage range.