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Old 10th February 2014, 05:48 PM   #11
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Bigtim1966 View Post
I added the 1 meg resistor from above but it didn't seem to help. Have some parts on order to build the instrumentation amp CCT from the data sheet, hoping this will work.
maybe the chip does not have hi-z input
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Old 10th February 2014, 07:56 PM   #12
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Quote:
Originally Posted by Zero D View Post
@ Bigtim1966

The 1 Meg resistor should have worked ? If you could draw a circuit of what you've done so far, & what you're proposing, it would assist us better in helping you
We've no idea what circuit he's using, and assuming it's too low an input impedance?, then reducing it further (even if only slightly) by adding an extra 1meg in parallel will only make it worse.

Presumably you were assuming he's blindly copied the partial circuit from the datasheet?, which makes no pretence of been a full working circuit.

As you say, and as always, the OP need to post EXACTLY what circuit he's built.
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Old 10th February 2014, 08:11 PM   #13
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I will do a sketch, but in fact Nigel is right, my receiver circuit is nothing more than the figure from the data sheet, so just the ina134 and a couple of 1uf caps on the +\- supplies. And my next step was to add the two additional amps to make the instrument amplifier from the data sheet.

I have a bad feeling I am out of my depth if there is additional circuitry to be designed around these basic figures from the datasheet
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Old 10th February 2014, 08:49 PM   #14
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try balanced line driver
or take a look on ESP site
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Old 11th February 2014, 02:01 AM   #15
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Quote:
Originally Posted by Nigel Goodwin

assuming it's too low an input impedance?, then reducing it further (even if only slightly) by adding an extra 1meg in parallel will only make it worse.
Hi, i was talking about putting a 1 Meg in parallel with the Positive input of the Buffer OpAmp he said he was going to use, which would better match the guitar. I wasn't suggesting connecting the 1 Meg to the ina134 circiut.

@ Bigtim1966

See my screenie for more details. Power supply connections not shown, as i'm guessing you know how to do that? ina134 details not shown, as you have the PDF for those.
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Old 11th February 2014, 05:57 AM   #16
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in this case it is my understanding the 1M is a safety resistor, should input be disconnected

or it could be a pulldown resistor if impedance is too high ... but you can't raise it, unless maybe you also attenuate the signal

and maybe the chip could be only 50K (still missing a number ?)
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Old 11th February 2014, 07:58 AM   #17
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It is not possible to do what you want with INA134 because the resistors are already inside the chip, therefore the input impedance will always be 25k or less.

You need to make a differential amplifier based around an op-amp with 1 meg resistors on the inverting side and 510k on the noninverting side. The problem here is that in the balanced mode the performance will be compromised by the high value resistors.
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Old 11th February 2014, 08:24 AM   #18
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Originally Posted by Bigtim1966 View Post
I have a bad feeling I am out of my depth if there is additional circuitry to be designed around these basic figures from the datasheet
Datasheets generally don't give you working practical circuits, they only give you the absolute basic minimum you require in order to design a working circuit - although there are occasionally full working examples.
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Old 11th February 2014, 06:36 PM   #19
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Originally Posted by Bigtim1966 View Post
I wondered if the basic receiver CCT I am using is not suited to the hi-z guitar output or if I am simply using too basic a receiver CCT.
you got the answer
what now ?
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Old 11th February 2014, 09:13 PM   #20
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Richie has I think hit the nail on the head. The INA134 internal resistors are providing too low an input impedance for the guitar.

With this in mind I built the cct which I hope in in the pdf attached to this message.
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