C
[email protected]
- Jan 1, 1970
- 0
Dear All,
I've been bitten by a slower rise and fall time than I was expecting
at the multiplexer ouput O in the following circuit, what have I
missed, please? You'll need to copy and paste into a constant width
font to see my ASCII, dare I say it, art
------
| U1 |
-------|A |
| | U2
-------1k--------|I1 | |\
| | | O|--------|+\
| ---1k--------|I2 | | >-----
| | | | | | |--|-/ |
1.1V0.1V 15n 15n ------ | |/ |
| | | | | |
--- --- --- --- |---91k--|
| |
|---27p--|
39k
|
---
U1 is a simplified representation of an MC14067 analogue multiplexer.
Two DC inputs have low pass filters of 1k/15nF.
The selection of these inputs is controlled by logic input A.
The multiplexer output drives an MCP601 CMOS opamp with a gain of 3.3'
and a low pass characteristic with 27pF in the feedback path.
When switching between the inputs I am finding a larger rise and fall
time at the multiplexer output, ~250us, than I am expecting.
I would have expected small rise and fall times (us at most) due to
the multiplexer's on resistance (say 1k) and output capacitance
(100pF).
Note I say at the multiplexer output. The inputs I1 & I2 are
unaffected during switching so I don't believe I'm seeing a charge
transfer effect. The effect is symmetrical so I don't believe I'm
seeing crosstalk from the control input.
....and what can I do about it?
regards & thanks for looking,
Colin Smith
I've been bitten by a slower rise and fall time than I was expecting
at the multiplexer ouput O in the following circuit, what have I
missed, please? You'll need to copy and paste into a constant width
font to see my ASCII, dare I say it, art
------
| U1 |
-------|A |
| | U2
-------1k--------|I1 | |\
| | | O|--------|+\
| ---1k--------|I2 | | >-----
| | | | | | |--|-/ |
1.1V0.1V 15n 15n ------ | |/ |
| | | | | |
--- --- --- --- |---91k--|
| |
|---27p--|
39k
|
---
U1 is a simplified representation of an MC14067 analogue multiplexer.
Two DC inputs have low pass filters of 1k/15nF.
The selection of these inputs is controlled by logic input A.
The multiplexer output drives an MCP601 CMOS opamp with a gain of 3.3'
and a low pass characteristic with 27pF in the feedback path.
When switching between the inputs I am finding a larger rise and fall
time at the multiplexer output, ~250us, than I am expecting.
I would have expected small rise and fall times (us at most) due to
the multiplexer's on resistance (say 1k) and output capacitance
(100pF).
Note I say at the multiplexer output. The inputs I1 & I2 are
unaffected during switching so I don't believe I'm seeing a charge
transfer effect. The effect is symmetrical so I don't believe I'm
seeing crosstalk from the control input.
....and what can I do about it?
regards & thanks for looking,
Colin Smith