Precision resistors

[eapavant]

Why not just buy bulk and get a lcr?

 

[carlosfm]

A very good reason for matching resistors is when building a differential amp.

From Analog Devices' AN734 appnote:

"Difference amplifiers are commonly used in high accuracy circuits to improve the common-mode rejection ratio, typically known as CMRR.
For this type of application, CMRR depends upon how tightly matched resistors are used; poorly matched resis
tors result in a low value of CMRR.
...
For example, when R7 = R6 = 10 k, and R2 = R4 = 1 k, and error = 0.1%, CMRR improves to better than 80 dB.
...
Select resistors that have much tighter tolerance and accuracy. The more closely they are matched, the better the CMRR. For example, if a CMRR of 90 dB is needed, then match resistors to approximately 0.02%."

 

[neutron7]

Bridge amp are not too critical, but the parallel output recommended for National chip amps is.

I simulated a paralleled opamp arrangement, basically a dual LM3886 circuit scaled by a factor of 10.

blue and khaki are with 1% error in gain
red and green are with 0.1%.

The National PA100 application note emphasises minimising dc offset, going as far as using servos, but the gain error is the major problem.

I wonder how safe the paralleled circuit is at both low frequencies where the dc block electrolytic would be hard to match and at high frequencies where the device bandwidth spread will start to become significant.

When errors of just 0.1% become important, even the thermal layout of the power amp pcb becomes critical.

how close are the Chips themselves are they closer than 0.1% gain ? if they are not you will be rolling the dice. unless you buy a whole bunch of LM chips and make a test rig to gain match them (can you even get a to220/11 socket?

you can put the DC blocking cap right on the input so both chips will see exactly the same signal.

 

[sam9]

The topic of the thread was "where do I buy 0.1% resistors" not "do you think I need 0.1% resistors".

To the original question - in addition to Mouser, I believe Michael Percy stocks such.
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When I've screened 5% resistors that come on tape the actaual variability is much less than that (+/-1-2%) and when I've screened 1% types it is sometimes difficult to even see the variation with in the limits of a typical multimeter. Extraploating from that rule, I suspect that using 0.1% results in mathing somwhat tighter than 0.1%. Looking at the app note, the prospect of keeping four LM3886's operating with nearly identical gain and current, using the closest tolerance resitors available.

I.e., not withstanding my previous comment 0.1% resitors used per the app note sound like a reasonmable precaution.

 

[safetyman]

Noob here....So what if I buy 5% resistors and match their value to exactly to each other on a normal multimeter, is that good enough or I still need the 1% or 0.1% and not having to match them.

I have thought that former would be good enough. Correct me please if I ma wrong or am not following waht has been posted especially Carlos's post.

Thanks.

 

[Panelhead]

Matching

Noob here....So what if I buy 5% resistors and match their value to exactly to each other on a normal multimeter, is that good enough or I still need the 1% or 0.1% and not having to match them.

I have thought that former would be good enough. Correct me please if I ma wrong or am not following waht has been posted especially Carlos's post.

Thanks.

Most digital VOM meters are 3 1/2 digit. This is not enough resolution to match up resistors to 0.1%.
If you spring for a 4 1/2 digit meter this is no problem. I am a big believer in resistor matching. Sometimes resistors measure off from printed value. Makes me wonder if some of the projects built in the past had an off value resistor in a critical position.
Those brown bodied Dales in 0.1% are very nice resistors. If Mouser stocks that is great.


George

 

[eapavant]

Newark is also a good choice. They seem to have better prices than Digikey or Mouser.

 

[carlmart]

Newark is also a good choice. They seem to have better prices than Digikey or Mouser.

That might be the case on some specific part. In general you can get better deals from Mouser, except on Panasonic caps.

Is there a specific line you are talking about?