SPDIF driver - 74HCU04 output impedance?

Adrian · 22nd July 2005, 04:21 PM

Hi,

I'm looking at a simple capacitively coupled spdif driver using parallel 74hcu04s. In calculating the resistor values to get the correct ouput level and impedance is it worthwhile taking account of the 74hcu04 output imdedance - if so could someone suggest the value to use [can't find one on the relavent data sheets except a generic one from from Fairchild which suggests HC normal outputs are in the region of 40-50 ohms - http://www.fairchildsemi.com/an/AN/AN-393.pdf.]

Thanks.

Jocko Homo · 22nd July 2005, 05:05 PM

40-50 ohms may be a bit high, but...............

Don't use parallel gates. Maybe if they are in separate packages, with ferrites to isolate them. Maybe.

But, yes, you do need to take into account the output Z of the chips. Something that a lot of alledged "high-end" designers never learned. Or if they did, neglected to take into account.

BTW.......I would measure it. A resistor and DVM will be enough to get you close.

Jocko

Elso Kwak · 22nd July 2005, 07:12 PM

An opamp with feedback will have almost zero output impedance. Wait a minute Jocko, did I say f-e-e-d-b-a-c-k?

Bricolo · 22nd July 2005, 07:52 PM

round 1!

Adrian · 22nd July 2005, 07:54 PM

Thanks Jocko. Can you confirm what the problem with parallel gates is?

Adrian

Elso Kwak · 22nd July 2005, 08:00 PM

Quote:

Originally posted by Bricolo
round 1!

Bricolo, you are sure you are in the right place?

We don't have a gong here.

Bricolo · 22nd July 2005, 11:15 PM

is it better now?

Adrian · 22nd July 2005, 11:24 PM

Meanwhile, back at the original question - any info on why not to use parallel gates would be appreciated.

Jocko Homo · 23rd July 2005, 12:05 AM

I didn't see him mention feedback. Did you? And how much open-loop gain do you think that one has???? Enough to lower the Z to zero ohms??

Anyway........if you have 6 inverters in one chip drawing six times the current, then the di/dt should be 6 times higher. Only one ground pin on the IC.......all that current in phase through it.....................you fill in the missing pieces.

Jocko

Guido Tent · 23rd July 2005, 09:23 PM

Quote:

Originally posted by Jocko Homo
I didn't see him mention feedback. Did you? And how much open-loop gain do you think that one has???? Enough to lower the Z to zero ohms??

Anyway........if you have 6 inverters in one chip drawing six times the current, then the di/dt should be 6 times higher. Only one ground pin on the IC.......all that current in phase through it.....................you fill in the missing pieces.

Jocko

Hi

Jocko is right both in terms of Zout as well as on advising on paralleling inverters. Additional note: Zout is only defined at the transfer of 0 to 1, in all other states the output impedance is high.

cheers

22nd July 2005, 04:21 PM	#1
Adrian diyAudio Member Join Date: Jul 2003 Location: .	SPDIF driver - 74HCU04 output impedance? Hi, I'm looking at a simple capacitively coupled spdif driver using parallel 74hcu04s. In calculating the resistor values to get the correct ouput level and impedance is it worthwhile taking account of the 74hcu04 output imdedance - if so could someone suggest the value to use [can't find one on the relavent data sheets except a generic one from from Fairchild which suggests HC normal outputs are in the region of 40-50 ohms - http://www.fairchildsemi.com/an/AN/AN-393.pdf.] Thanks.

22nd July 2005, 07:52 PM	#4
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	round 1! __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

22nd July 2005, 11:15 PM	#7
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	is it better now? __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
idiots guide to impedance + spdif measurement	woodturner-fran	Digital Line Level	7	20th October 2008 06:04 PM
3C24 in a SE output stage - output transformer impedance recommendation !!!	aldovan	Tubes / Valves	15	6th September 2008 10:18 AM
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms	kelticwizard	Everything Else	11	25th March 2007 05:17 AM
74xx74 a good digital output (AES-EBU/SPDIF)driver?	Dr.H	Digital Source	3	7th April 2005 03:10 PM
Using 74HCU04 in a headphone amp	Tomo	Headphone Systems	6	26th September 2004 08:23 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

22nd July 2005, 05:05 PM	#2
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	40-50 ohms may be a bit high, but............... Don't use parallel gates. Maybe if they are in separate packages, with ferrites to isolate them. Maybe. But, yes, you do need to take into account the output Z of the chips. Something that a lot of alledged "high-end" designers never learned. Or if they did, neglected to take into account. BTW.......I would measure it. A resistor and DVM will be enough to get you close. Jocko

22nd July 2005, 07:12 PM	#3
Elso Kwak Banned Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao	An opamp with feedback will have almost zero output impedance. Wait a minute Jocko, did I say f-e-e-d-b-a-c-k?

22nd July 2005, 07:54 PM	#5
Adrian diyAudio Member Join Date: Jul 2003 Location: .	Thanks Jocko. Can you confirm what the problem with parallel gates is? Adrian

22nd July 2005, 11:24 PM	#8
Adrian diyAudio Member Join Date: Jul 2003 Location: .	Meanwhile, back at the original question - any info on why not to use parallel gates would be appreciated.

23rd July 2005, 12:05 AM	#9
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	I didn't see him mention feedback. Did you? And how much open-loop gain do you think that one has???? Enough to lower the Z to zero ohms?? Anyway........if you have 6 inverters in one chip drawing six times the current, then the di/dt should be 6 times higher. Only one ground pin on the IC.......all that current in phase through it.....................you fill in the missing pieces. Jocko

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read