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What is the best way to use mono input to both ADC's
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Wed Jul 01, 2015 3:19 am    Post subject: What is the best way to use mono input to both ADC's Reply with quote

I am using a mono input buffer/filter circuit into the FV-1 which drives both the ADC's pin 1 and 2 of the Fv-1. Pins 1&2 are connected directly to each other.

I am now considering driving both inputs from the mono source but each of the ADC's having it's own DC decoupling from the mono input rather than being connected directly.


Having previously read that both inputs could be directly connected I have always used this method, may this cause a slight DC offset across the ADC's or is it perfectly fine to carry on with the 2 ADC's connected this way rather than keeping them separate. If I decouple them then it really is only a couple of extra components to add.
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Digital Larry



Joined: 12 Nov 2012
Posts: 266

PostPosted: Thu Jul 02, 2015 12:36 pm    Post subject: Reply with quote

I don't see any benefit to doing mono this way. Why not just connect one input and then read from ADCL when you need to?
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amz-fx



Joined: 06 Sep 2006
Posts: 31
Location: Louisiana, USA

PostPosted: Wed Jul 08, 2015 10:35 pm    Post subject: Reply with quote

Digital Larry wrote:
I don't see any benefit to doing mono this way. Why not just connect one input and then read from ADCL when you need to?


I would tie the unused ADCR to ground through a 100n capacitor just so a high impedance input isn't left waving in the breeze like an antenna. Probably not important as long as you are not reading the input, but a good practice nonetheless.

regards, Jack
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Thu Jul 09, 2015 11:28 am    Post subject: Reply with quote

I am using Both inputs, reading them both at 0.5 then summing them together then writing to a location to use later. I am not sure if this is correct but doing this seems to give a better s/n ratio and more headroom compared to reading one input at say 0.8. What I am really asking though is that having tied both ADC's directly together may a small DC offset be present or is it better to decouple both inputs with a C/R network first.

The datasheet mentions internal 1/2 Vref on both ADC's but no mention of whether they are internally decoupled.
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slacker



Joined: 01 Feb 2011
Posts: 115

PostPosted: Sat Jul 11, 2015 8:23 am    Post subject: Reply with quote

There shouldn't be any DC offset, the inputs are high pass filtered which would remove any DC that was present.
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Mon Jul 13, 2015 1:06 pm    Post subject: Reply with quote

slacker wrote:
There shouldn't be any DC offset, the inputs are high pass filtered which would remove any DC that was present.


After reading the Datasheet (again) I not sure the inputs are high pass filtered, the datasheet shows external high pass circuit on each input of course but this is not what I am really referring too.

This is from the datasheet.
'The inputs should be conducted through coupling capacitors, as the inputs are internally biased to a VDD/2 potential'

So I believe that answers my question really.
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Sweetalk



Joined: 15 Oct 2009
Posts: 95

PostPosted: Tue Jul 14, 2015 5:35 am    Post subject: Reply with quote

If you don't bypass the inputs there's Vcc/2 on each one. I'm tying both together and reading them both also.
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Wed Jul 15, 2015 12:25 am    Post subject: Reply with quote

Sweetalk wrote:
If you don't bypass the inputs there's Vcc/2 on each one. I'm tying both together and reading them both also.


That is the method I have always used, with both inputs directly linked together. I was just wondering if this was good practice or may introduce a small hum. Would separating the inputs through separate dc blocking and LP filter networks benefit at all?

;read both inputs and store in 'dry' register
rdax adcl,0.5
rdax adcr,0.5
wrax dry, 0
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Sweetalk



Joined: 15 Oct 2009
Posts: 95

PostPosted: Wed Jul 15, 2015 4:37 am    Post subject: Reply with quote

ice-nine wrote:
Sweetalk wrote:
If you don't bypass the inputs there's Vcc/2 on each one. I'm tying both together and reading them both also.


That is the method I have always used, with both inputs directly linked together. I was just wondering if this was good practice or may introduce a small hum. Would separating the inputs through separate dc blocking and LP filter networks benefit at all?

;read both inputs and store in 'dry' register
rdax adcl,0.5
rdax adcr,0.5
wrax dry, 0


That's a good question and I have the same one. We'll have to try both optinos and check the results...
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Wed Jul 15, 2015 8:30 am    Post subject: Reply with quote

Sweetalk wrote:
ice-nine wrote:
Sweetalk wrote:
If you don't bypass the inputs there's Vcc/2 on each one. I'm tying both together and reading them both also.


That is the method I have always used, with both inputs directly linked together. I was just wondering if this was good practice or may introduce a small hum. Would separating the inputs through separate dc blocking and LP filter networks benefit at all?

;read both inputs and store in 'dry' register
rdax adcl,0.5
rdax adcr,0.5
wrax dry, 0


That's a good question and I have the same one. We'll have to try both optinos and check the results...


I will find out soon enough as I have send new pcb's to fab that can be stuffed either way. I will post results when they come back.
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Sweetalk



Joined: 15 Oct 2009
Posts: 95

PostPosted: Fri Jul 17, 2015 4:11 am    Post subject: Reply with quote

ice-nine wrote:
Sweetalk wrote:
ice-nine wrote:
Sweetalk wrote:
If you don't bypass the inputs there's Vcc/2 on each one. I'm tying both together and reading them both also.


That is the method I have always used, with both inputs directly linked together. I was just wondering if this was good practice or may introduce a small hum. Would separating the inputs through separate dc blocking and LP filter networks benefit at all?

;read both inputs and store in 'dry' register
rdax adcl,0.5
rdax adcr,0.5
wrax dry, 0


That's a good question and I have the same one. We'll have to try both optinos and check the results...


I will find out soon enough as I have send new pcb's to fab that can be stuffed either way. I will post results when they come back.


Great!!
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Sandrine



Joined: 12 Mar 2015
Posts: 16
Location: BC Canada

PostPosted: Thu Aug 06, 2015 7:29 am    Post subject: Reply with quote

I would use a cap and resistor on each input. The variance in DC offset could cause noise, to varying degrees.
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Sun Aug 09, 2015 5:24 am    Post subject: Reply with quote

Sandrine wrote:
I would use a cap and resistor on each input. The variance in DC offset could cause noise, to varying degrees.


I made a last minute revision to the boards and added the option to be able to use either input configuration by not fitting some components and joining the inputs or to fit all the components for separate input de coupling.

I have built the joint input layout first and feel it is noise free, next build will be separate inputs and I will compare the two builds for any differences.

Picture of the first build

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Sweetalk



Joined: 15 Oct 2009
Posts: 95

PostPosted: Wed Aug 12, 2015 4:40 am    Post subject: Reply with quote

ice-nine wrote:
Sandrine wrote:
I would use a cap and resistor on each input. The variance in DC offset could cause noise, to varying degrees.


I made a last minute revision to the boards and added the option to be able to use either input configuration by not fitting some components and joining the inputs or to fit all the components for separate input de coupling.

I have built the joint input layout first and feel it is noise free, next build will be separate inputs and I will compare the two builds for any differences.

Picture of the first build



Great!!, what anti-alias filter are you using?.
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ice-nine



Joined: 24 May 2012
Posts: 119

PostPosted: Fri Aug 14, 2015 11:17 am    Post subject: Reply with quote

Sweetalk wrote:


Great!!, what anti-alias filter are you using?.


Hi Sweetalk, In front of the FV-1 is a sallen-key filter which at 8khz.
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