Aside from the suggestions given consider the following... Use a Polyester capacitor. Remember, capacitors, although designed for energy storage do exhibit leakage. The lowest leakage variety are Polyester... PCB layout also contributes. Ensure that the board is clean and isolate the capacitor connections as best as possible from GND and POWER tracks. Also please consider the use of a guard ring around the Ch pin of the LM398, TOP and BOTTOM of board. Read the application notes carefully for the device. All that I have mentioned is contained within the LM398 datasheet... Richard Prosser wrote: > > I can only suggest you check your supply & logic voltages, connections etc. > The device needs both a +ve and a -ve supply (from 5V to 18V) according to > the data sheet. Without both supplies it won't work. Also the logic control > input reference should be tied to the ground - i.e. midway between the +ve > and -ve. If this midpoint is not connected to the bottom end of the cap then > the cap might be regarded as floating - and not store any charge. > How about measuring the voltage across the cap. directly? Using one of your > larger caps you should get an initial voltage & be able to watch it decay > through the meter. > Or you could try precharging the cap (via a resistor and in hold mode) and > see what the output is doing. > Either of these should at least give an indication as to which side of the > circuit is not performing. > > Sorry, Can't suggest much more than this. > Richard > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > Swichtec Power Systems Limited, | Phone: (++64) 3 343-3314 > 39 Princess St, PO Box 11-188, | Fax: (++64) 3 348-8871 > Christchurch, New Zealand. | Email: rprosser@swichtec.co.nz > Visit us at http://www.swichtec.co.nz > +++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > -----Original Message----- > > From: Lauw Lim Un Tung [mailto:tung@MITRA.NET.ID] > > Sent: Thursday, May 27, 1999 3:13 PM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Sample & Hold Circuit (fwd) > > > > > > any idea for this problem? > > > > > > > ---------- Forwarded message ---------- > > > Date: Mon, 24 May 1999 12:27:48 +0700 (GMT+0700) > > > From: Tan Sing > > > To: Lauw Lim Un Tung > > > Subject: Re: Sample & Hold Circuit (fwd) > > > > > > Have you checked if the impedance of your multimeter is > > not affecting > > > the circuit ? > > > > > > Try to buffer the output with a voltage follower. > > > > > > Best regards, > > > Alexandre Guimaraes > > > alexg@iis.com.br > > > > > > -----Mensagem original----- > > > De: Tan Sing > > > Para: Multiple recipients of list EDESIGN > > > > > Data: Domingo, 23 de Maio de 1999 06:26 > > > Assunto: Sample & Hold Circuit > > > > > > >Dear Netters, > > > >I am using LF398 (DIP8) for my sample & hold circuit. In > > the datasheet, it > > > >is mentioned that the output voltage in hold mode will > > remain quite a long > > > >time. Unfortunately when I used it in my circuit, the > > output voltage drops > > > >very quickly in hold mode. I tried to change the capacitor > > from 1 nF, 10nF, > > > >100nF, 1uF, 10uF to 100uF. from ceramic type, > > electrolytic, to tantalum > > > >capacitor. But the symptoms remains the same. I tried to > > change several > > > >ICs, but the result were also the same. In the datasheet, > > it says that > > > >using 1 uF will have the output voltage drop of 5 mV/min. > > But This doesn't > > > >happen in my circuit. I just used the typical application > > of LF398, and > > > >directly measure the output voltage using a digital multimeter. > > > >I wonder what could possibly be the cause of this symptoms > > ??? Because the > > > >circuit application just consists of 1 IC and 1 cap. I > > have replace them > > > >with some capacitor and some ICs, but it seems it doesn't help. > > > >I used Philips Semiconductor DIP8 LF398, and also National > > Semiconductor > > > >DIP8 and Metal Can Package LF398. The result was the same. > > > > > > > >Is there anybody who could give me any suggestions ??? > > > > > > > >Thank you. > > > > > > Nino -- ****************************************************** * Antonio (Nino) Benci * * Electronic Services Manager * * Monash University - Dept of Physics * * Wellington Rd, Clayton. 3168 * * Victoria, Australia. * * TEL - 61 3 9905 3649, FAX - 61 3 9905 3637 * * Mobile - 0414 764 763 (private and ah only) * * EMAIL - nino.benci@sci.monash.edu.au (work) * * - fleatech@excite.com (private) * * WWW - www.physics.monash.edu.au/ * ******************************************************