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vdd/ground connection in mixed circuit

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John Xu

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vdd ground

Hi,
I have a question on the mixed signal circuit. We are designing a 155M limiting amplifier which consists of the cascade of amplifiers and an output buffer.We treat the cascade amplifiers as analog part and the output buffer as digital part because it works like a current switch. To isolate the crosstalk from the output buffer switching, we dedicated two supplies for the analog part and digital part, seperately.

But i have no idea for the vdd/ground connection for ptat current source whcih provide the reference bias current(~uA) for the tail current(16mA) of output buffer. What do you think the vdd/fround of this ptat current source should connect to?the analog vdd/gnd, or the digital vdd/gnd?Or the dedicated another vdd/gnd for this part?

Thanks in advance
 

IanP

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mixed circuit digital vdd

I would feed the output buffer from the digital Vdd through a RF-choke (almost as if you created separate Vdd) and connect a couple of decoupling caps at both sides of the RF-choke, and use as ground the digital GND ..
(assuming that the Analog and Digital grounds are connected somewhere ..)
Regards,
IanP
 

John Xu

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IanP said:
I would feed the output buffer from the digital Vdd through a RF-choke (almost as if you created separate Vdd) and connect a couple of decoupling caps at both sides of the RF-choke, and use as ground the digital GND ..
(assuming that the Analog and Digital grounds are connected somewhere ..)
Regards,
IanP
The ptat current source is a part of the output buffer, it provides the bias to the output buffer.But is an anlog circuit. I want to know its vdd/gnd should connected to digital vdd/gnd or to the analog vdd/gnd which is for cascaded amp stages?We have the concern if it is connected to digital vdd/gnd, the ptat current source will affected by the switching noise from buffer. But if it connedtd to the analog vdd/gnd, we not sure if it will degrade the isolatiion of crosstalk.

Can anyone help me?
 

John Xu

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Re: vdd/ground connection in mixed circuit, pls. help me.

can anyone pls. give me some advice????

Thanks in advance
 

Syukri

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as far as i know...
VDD and Gnd for both analog and digital is the same
as long as the VDD share the same value..
the if it's not some biasing cct needed.

Analog architechture manupulate the vdd to make operation eg amplifying

Digital use it for high and low notation

if your design still using transistor i think it's ok
 

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