Pulling down an AC signal to GND?

?

-

Guest
I need to selectively pull a 7Vrms AC Signal to 0V, using the output of a
comparator. It comes through 4.7k.

If it were DC I'd use a VN10LP N-Channel Enhancement-Mode MOSFET but that
conducts -ve voltage even when it's switched off so has problems with AC.

I've come up with a circuit involving two VN10s and diodes to ensure they
only act in one direction but I'm sure there must be more elegant way.

The supplies I have are +/- 18V and +5V.

I've also considered using an analogue switch.

I'm sure that there's a simpler way to do this, perhaps a N-Channel and a
P-Channel fet in parallel. (It would require that each doesn't conduct in
its reverse direction, which the VN10 does.)

Could somebody suggest something? Part names would be very handy :)

Thanks for any help.


-
 
- wrote:
I need to selectively pull a 7Vrms AC Signal to 0V, using the output of a
comparator. It comes through 4.7k.

If it were DC I'd use a VN10LP N-Channel Enhancement-Mode MOSFET but that
conducts -ve voltage even when it's switched off so has problems with AC.

I've come up with a circuit involving two VN10s and diodes to ensure they
only act in one direction but I'm sure there must be more elegant way.

The supplies I have are +/- 18V and +5V.

I've also considered using an analogue switch.

I'm sure that there's a simpler way to do this, perhaps a N-Channel and a
P-Channel fet in parallel. (It would require that each doesn't conduct in
its reverse direction, which the VN10 does.)

Could somebody suggest something? Part names would be very handy :)

Thanks for any help.


-
Click to expand...
Please view in a fixed-width font such as Courier.





4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
| 1
=== C= ---------------
| 4.7 x 2 pi x F
4.7K | min
18V>----/\/\---+
|
|
|
shunt c
+--- 10K |/
| >---/\/\--|
off-+ |\
e
|
---
gnd





- or -



4.7K
AC IN >---/\/\----+ +------>AC OUT
| |
----
opto bilateral sw
+-|>|-+
| |
| |
1.8K | |
18V -/\/\----+ +-+
|
|
|
on c
+--- 10K |/
| >---/\/\--|
off-+ |\
e
|
---
gnd
 
Fred Bloggs <nospam@nospam.com> wrote:
Please view in a fixed-width font such as Courier.





4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
| 1
=== C= ---------------
| 4.7 x 2 pi x F
4.7K | min
18V>----/\/\---+
|
|
|
shunt c
+--- 10K |/
| >---/\/\--|
off-+ |\
e
|
---
gnd
Click to expand...
Fred,

This circuit needs an ac-break in series with the 4.7k bias, otherwise
when the transistor is turned off, the circuit looks like:

4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
|
=== C
|
4.7K |
18V>----/\/\---+

which would be a resistive divider at the frequency of interest.

--
Rick
 
Rick wrote:
Fred Bloggs <nospam@nospam.com> wrote:

Please view in a fixed-width font such as Courier.





4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
| 1
=== C= ---------------
| 4.7 x 2 pi x F
4.7K | min
18V>----/\/\---+
|
|
|
shunt c
+--- 10K |/
| >---/\/\--|
off-+ |\
e
|
---
gnd



Fred,

This circuit needs an ac-break in series with the 4.7k bias, otherwise
when the transistor is turned off, the circuit looks like:

4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
|
=== C
|
4.7K |
18V>----/\/\---+

which would be a resistive divider at the frequency of interest.
Click to expand...
Right- call that a 6dB insertion loss:_)
 
- wrote:
I need to selectively pull a 7Vrms AC Signal to 0V, using the output of a
comparator. It comes through 4.7k.

If it were DC I'd use a VN10LP N-Channel Enhancement-Mode MOSFET but that
conducts -ve voltage even when it's switched off so has problems with AC.

I've come up with a circuit involving two VN10s and diodes to ensure they
only act in one direction but I'm sure there must be more elegant way.

The supplies I have are +/- 18V and +5V.

I've also considered using an analogue switch.

I'm sure that there's a simpler way to do this, perhaps a N-Channel and a
P-Channel fet in parallel. (It would require that each doesn't conduct in
its reverse direction, which the VN10 does.)

Could somebody suggest something? Part names would be very handy :)

Thanks for any help.

-
Click to expand...
You didn't say how fast you need to be able to enable and disable this
grounding. There are opto isolators with paris of fets it the switch
section that can do this when you power an LED. But they don't switch
very quickly.

Here is the data sheet from one of these sold by Digikey with a 200
ohm on resistance:
http://www.fairchildsemi.com/ds/H1/H11F1.pdf
--
John Popelish
 
Rick, a choke in series with the lower 4.7K should fix this. As long as it
won't series resonate with the cap.

Regards, Joerg
http://www.analogconsultants.com
 
Joerg <notthisjoergsch@removethispacbell.net> wrote:
Rick, a choke in series with the lower 4.7K should fix this. As long as it
won't series resonate with the cap.
Click to expand...
Thanks, Joerg. The ac-break I mentioned is just a special choke - an
ideal one with infinite inductance.

In fact, there's no reason why the choke you mentioned won't resonate
with the capacitor (when the transistor is off and hence effectively
not there). At resonance, the +jwL of the inductor is cancelled by the
-j/(wC) of the capacitor and we're back to a resistive divider. Away
from resonance, the series combination of the C, L and 4.7k will have to
look high-impedance compared to the 4.7k in the signal path, otherwise
you will still get some attenuation of the signal.


--
Rick
 
In article <408cf7f5$0$31712$fa0fcedb@lovejoy.zen.co.uk>,
- <Reply@Thread.pls> wrote:
I need to selectively pull a 7Vrms AC Signal to 0V, using the
[snip]
Could somebody suggest something? Part names would be very handy
:)
Click to expand...
Farnell part numbers below <http://www.farnell.com>
for more data.

Part Num 326-689 is an optocoupler with a bilateral
fet output, switchable resistance range is 200 ohms
to 300 megohms.

Part Num 316-8773 is a photoresistive optocoupler,
resistance range is 500 ohms to 500k.

The IR PVA-series of PhotoMOS switches may also be
suitable, eg, Farnell 331-120, etc.

--
Tony Williams.
 
Marvellous,

The bilateral-FET-output optocoupler idea looks most promising.

Thanks everyone!
 
- wrote:
Marvellous,

The bilateral-FET-output optocoupler idea looks most promising.

Thanks everyone!
Click to expand...
If you put two in series and drive them alternately, with the signal
entering the series pair, and the output between them, (make a single
pole double throw switch) you can get even better grounding of your
signal.

--
John Popelish
 
Fred Bloggs wrote:
- wrote:

I need to selectively pull a 7Vrms AC Signal to 0V, using the output of a
comparator. It comes through 4.7k.

If it were DC I'd use a VN10LP N-Channel Enhancement-Mode MOSFET but that
conducts -ve voltage even when it's switched off so has problems with AC.

I've come up with a circuit involving two VN10s and diodes to ensure they
only act in one direction but I'm sure there must be more elegant way.

The supplies I have are +/- 18V and +5V.

I've also considered using an analogue switch.

I'm sure that there's a simpler way to do this, perhaps a N-Channel and a
P-Channel fet in parallel. (It would require that each doesn't conduct in
its reverse direction, which the VN10 does.)

Could somebody suggest something? Part names would be very handy :)

Thanks for any help.


-



Please view in a fixed-width font such as Courier.





4.7K
AC IN >---/\/\----+--------->AC OUT
|
|
| 1
=== C= ---------------
| 4.7 x 2 pi x F
4.7K | min
18V>----/\/\---+
|
|
|
shunt c
+--- 10K |/
| >---/\/\--|
off-+ |\
e
|
---
gnd
Click to expand...
I guess we don't care about the 18V glitch that blasts out the output
when you switch the shunt????
mike


--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
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Yaesu FTV901R Transverter, 30pS pulser
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http://www.geocities.com/SiliconValley/Monitor/4710/
 
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