Sample and Hold Circuit Help!!

Thread Starter

syee10

Joined Mar 26, 2010
59
Hi all,

I am designing a simple sample and hold circuit where the input is a sinusoidal analog input and it is control by a TTL compatible square wave (waveform A). The result waveform i should get is waveform B in the attach file but i tried a millions time i still cant get the same waveform. Someone there can help me out? I had attached all the file in the attachment..
 

Attachments

Ron H

Joined Apr 14, 2005
7,014
Hi all,

I am designing a simple sample and hold circuit where the input is a sinusoidal analog input and it is control by a TTL compatible square wave (waveform A). The result waveform i should get is waveform B in the attach file but i tried a millions time i still cant get the same waveform. Someone there can help me out? I had attached all the file in the attachment..
I can make a sample and hold (track and hold, more accurately) using your circuit, with a couple of mods. On mine, though, when the sample pulse goes high, the output follows the input instead of being clipped at ground like your output waveform.
 

Thread Starter

syee10

Joined Mar 26, 2010
59
Ron..

Your result is exactly what i want..anyway i cant open the file you attached here..can you send me the LF356.sub? Thank you =)
 

Ron H

Joined Apr 14, 2005
7,014
Copy and paste this into Notepad, and save it as ...\lib\sub\LF356.sub.
Rich (BB code):
* connections:    non-inverting input
*                 |   inverting input
*                 |   |   positive power supply
*                 |   |   |   negative power supply
*                 |   |   |   |   output
*                 |   |   |   |   |
*                 |   |   |   |   |
.SUBCKT LF356     1   2  99  50  28
*
*Features:
*Low input bias current =             30pA
*Low input offset current =            3pA
*High input impedance =              1Tohm
*Low input offset voltage =            1mV
*
****************INPUT STAGE************** 
*
IOS 2 1 3P
*^Input offset current
R1 1 3 1E12
R2 3 2 1E12
I1 99 4 100U
J1 5 2 4 JX
J2 6 7 4 JX
R3 5 50 20K
R4 6 50 20K
*Fp2=20 MHz
C4 5 6 1.9894E-13
*
***********COMMON MODE EFFECT***********
*
I2 99 50 4.65MA
*^Quiescent supply current
EOS 7 1 POLY(1) 16 49 3E-3 1
*Input offset voltage.^
R8 99 49 50K
R9 49 50 50K
*
*********OUTPUT VOLTAGE LIMITING********
V2 99 8 2.63
D1 9 8 DX
D2 10 9 DX
V3 10 50 2.63
*
**************SECOND STAGE**************
*
EH 99 98 99 49 1
F1 9 98 POLY(1) VA3 0 0 0 1.5944E7
G1 98 9 5 6 2E-3
R5 98 9 100MEG
VA3 9 11 0
*Fp1=31.96 HZ
C3 98 11 49.9798P
*
*********COMMON-MODE ZERO STAGE*********
*
G4 98 16 3 49 1E-8
L2 98 17 530.52M
R13 17 16 1K
*
**************OUTPUT STAGE**************
*
F6  99 50 VA7 1
F5  99 23 VA8 1
D5  21 23 DX
VA7 99 21 0
D6  23 99 DX
E1  99 26 99 9 1
VA8 26 27 0
R16 27 28 20
V5  28 25 -.25V
D4  25  9 DX
V4  24 28 -.25V
D3   9 24 DX
*
***************MODELS USED**************
*
.MODEL DX D(IS=1E-15)
.MODEL JX PJF(BETA=1.25E-5 VTO=-2.00 IS=30E-12)
*
.ENDS
 

Thread Starter

syee10

Joined Mar 26, 2010
59
Ron..

Thanks ron..actually i am using Labcenter Proteus to do my simulation..but i already tried to use LTspice and it works.. but i wonder what parameter you change to get the sample waveform? I only can get the hold waveform with my Proteus.. so i need to know which parameter i can change to get sample waveform.. the switch is not the active device rite?
 

Ron H

Joined Apr 14, 2005
7,014
Ron..

Thanks ron..actually i am using Labcenter Proteus to do my simulation..but i already tried to use LTspice and it works.. but i wonder what parameter you change to get the sample waveform? I only can get the hold waveform with my Proteus.. so i need to know which parameter i can change to get sample waveform.. the switch is not the active device rite?
I'm not familiar with Proteus. Maybe one of the other members can help.
 

Thread Starter

syee10

Joined Mar 26, 2010
59
Ron,

Now i have a similar question about it. What if i change the input voltage source to a input current source?the result waveform i want to get is exactly the same with the previous input voltage source. I tried to replace the input voltage source already but i still cannot get the correct output sample and hold waveform. The output waveform of the current i get is continuous zero value. So do you have any idea? Please reply me asap if possible. Thank you
 

Attachments

Ron H

Joined Apr 14, 2005
7,014
Why do you want to use current as an input? Where is the current coming from?
Current will not flow into an open circuit (the input of U1). Remember Ohms law: V=I*R. If R is infinite, V will be infinite.
 

ifixit

Joined Nov 20, 2008
650
Your sample and hold circuit needs a voltage input to work. To convert a current source back to a voltage, put a resistor of a suitable value from the U1 + input to ground.

Regards,
Ifixit
 

Ron H

Joined Apr 14, 2005
7,014
Your sample and hold circuit needs a voltage input to work. To convert a current source back to a voltage, put a resistor of a suitable value from the U1 + input to ground.

Regards,
Ifixit
Or dump it into the inverting input of a feedback amplifier.
 
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