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IamSmooth

Sat Mar 15 2014, 05:53PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I am trying to buy an opamp (2 or 4 channel) that has a very low temperature drift and low offset voltage. I want a single voltage supply. I have the LMC6064, but I need a lower voltage drift. I thought the LT1114 would be a good choice. However, when I swap this chip in for the LMC6064 the output is totally different.

I thought it is a single/dual-supply chip. I'm supplying both with 5v to V+ and ground to V-. What am I missing here? I don't think the schematic of what I'm doing matters. The fact is that when I swap the two chips I get two different outputs when they should be the same.

Sulaiman

Sat Mar 15 2014, 06:07PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

possibly due to the input common mode range not including the negative supply
(0v if single supply)
see the datasheet diagram

"Common Mode Range and
Voltage Swing with Respect to
Supply Voltages"

i.e. if you reference your circuit to 0v you need a
'rail-to-rail input range, or at least to the -ve rail

the LMC6064 datasheet indicates that
"Input Common-Mode Range Includes Vâˆ’"

IamSmooth

Sat Mar 15 2014, 06:10PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Just to clarify. I made a test circuit: a simple non-inverting opamp using 51k resistors for a gain of 2. My 6064 chip gives me 5v with a 2.5v input. The lt1114 gives me 4.32v.

Sulaiman, could you go into more detail on your answer with regards to the common mode range and swing. I am not familiar with the terminology.
What chip can I use?

Sulaiman

Sat Mar 15 2014, 06:18PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I'm not 'up' with the latest op-amps so I'd prefer someone else suggest,
but I'll have a look ...

op-amps commonly have + and - supply voltages with respect to circuit common/ground.
due to design/semiconductor considerations the signals move around the common point and usually the output can not reach the full positive or negative supply, those that can are called
'rail-to-rail' output

similarly the circuitry usually does not work when the inputs are at or near the supply voltages (e.g. 0v = -supply if single supply)
those that can are called rail-to-rail input capable.

You also have to bear in mind that an inverting amplifier with inputs near 0v cannot work as the output cannot go below 0v.

IamSmooth

Sat Mar 15 2014, 06:25PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

My test signal is a simple 2.5v dc input. My gain is 2x. The fact that one chip does 4.98 and the other 4.31 is a big difference. I plan on calling Linear technology on MonDAY and find out what they have to say.

Sulaiman

Sat Mar 15 2014, 07:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

can you sketch out your circuit diagram?
easier to comment.

IamSmooth

Sat Mar 15 2014, 07:36PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

This is the test circuit. The output is different with both chips. It should be the same. The input to the non-inverting is 2.5v DC. V+ is 5v. V- is gnd.

Sulaiman

Sat Mar 15 2014, 07:49PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If the input is 2.5v, gain = 2 (R1=R2), then the output should be 5v.
and as above, the output cannot go to 5v
look at the the graph
"Output Voltage Swing
vs Load Current"
it shows that the output cannot reach V+
with a light load you may get to within 0.7V of V+ .... 4.3V !!!

you have a few choices;
.. use an op-amp with 'rail-to-rail' inputs and output
.. design your circuit to not use voltages near the supplies (ground, V+)
.. use +/- supplies for the amplifier(s)

in precision systems it is common to see dc/dc converters used to
.. convert 5V (or other available low voltage dc) to +/- 5V, 12V etc
.. isolate the sensitive analogue circuitry from the 'noisy' logic supply,
by using an 'isolated' dc/dc converter
typically op-amps for gain and filtering feeding an A/D converter with a serial digital output via an opto-isolator to the main digital circuitry.

IamSmooth

Sat Mar 15 2014, 08:03PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Maybe my example (test circuit) is poor choice. Below is the actual circuit that I need

The input is 0.05v. This should give 0.05v on the output. I am getting 0.461v with this chip. I get 0.05v with my 6064 chip.

Simply swapping chips gives drastically different results.

Sulaiman

Sat Mar 15 2014, 08:14PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

"The input is 0.05v"

the lt1114 op-amp does not work correctly when the input or output is near 0V(V-) !

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