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what is the supply connection fo this output buffer?

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John Xu

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Jul 22, 2005
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I have a question for the vdd/gnd connection for the attached output buffer. Pls. see the attached schematic, this is the output stage, before stage is analog part. The function of this stage is a current switch, the tail current is ~16mA which is mirrored from a supply-independent current source. Considering the switching chracteristics of this output buffer, we dedicated two vvd/gnd domains to supply it. The analog part before is analog vdd/gnd, the output buffer is digital vdd/gnd.

But I have some puzzles for the vdd/gnd connection for the supply independent current source. Which vdd/gnd should i connect to? The substrate is coonected to analog gnd. The process is nwell p-sub light doply cmos. Our concerns is:
1.if it is connected to analog part, the digital gnd bounce will introduce gnd potential difference for the current mirror, which will lead to the current mirror error.

2.If it will connect to the digital vdd/gnd, the body effect of the M1 will also induce the error for the current mirror accuracy,because the substrate is connected to analog gnd

3.What ever it connected to the analog vdd/gnd, or digital vdd/gnd, M2 always have the body effect which will indroduce the inaccuray of tail current, thus degrade the eye-diagram of output.

Then, what should i connect it to??? Pls. give me some advice!!

Thanks in advance

How I would do it :

Connect the current mirror to analog ground (M1 and M2), so you are sure of a good mirroring without body effect.

The current through M1 and M2 will normally stay constant, so it it will not disturb the analog ground too much. (considering that at least one of the input pairs is on)

The current that is delivered to the input pair doesn't care about ground shifts, it is a very high impedant node.

The supply of the current switch (top connection of 50 Ohm) I would connect to digital VDD, because indeed, the switching current will cause some disturbance. (depending of the layout).

br, JT

    John Xu

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Yes. My question is why there's a series resistor connected to the source of M2? Just used to describe the substrate resistance?

qtarch said:
Yes. My question is why there's a series resistor connected to the source of M2? Just used to describe the substrate resistance?

This cicruit is refenced from some other one's design, i guess this resistor is a degeneration resistor which can increase teh lininearity of the tail current.

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