LT1021CCN8-5 2-Pole low pass Filtered Voltage Reference any advice  - Page 1
Forum section: electronics / general

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 02, 2025, 01:28:12 pm
content: HelloI know that LT1021 is not the best but does this circuit improve the performance ? or the performance gain is limited , that an AD provided circuitSome current source use low pass filter for Op Amp reference voltage and Ref50XX have the same pinout as LT1021 so I suppose this circuit can be cloned.Any advice ?RegardsOS

author: tszaboo
author_rank: Super Contributor
timestamp: April 02, 2025, 01:54:43 pm
content: This is a Sallen-Key low pass filter on the output. It will reduce high freq. noise coming from either the reference or it's power supply. Maybe it also improves the output drive.The tradeoff is offset voltage for example. You should set out goals, what to improve, and what characteristics is important.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2025, 05:10:31 pm
content: The filter helps against the higher frequency noise. It depends on the use case if that noise is relevant. It can matter with higher speed ADCs and DACs, but it would matter less for lower speed ADCs. Lower speed applications are also more effected from the lower frequency noise and may thus care less about the higher frequency noise.In some cases there is aliasing with the reference noise and thus way also the higher frequeny noise can effect a slow SD or similar ADC.If the circuit anyway needs something like a buffer or amplifier it is often possible to include a filter function with little extra effort and downsides.

author: mawyatt
author_rank: Super Contributor
timestamp: April 02, 2025, 05:36:13 pm
content: The Sallen-Key LPF has limited Stop-Band Attenuation due to the "bleed thru" from the capacitor between the resistors to the Op-Amp Output. Adding some DC bias current from the Op-Amp Output will lower the Op-Amp Output Z at higher frequencies and improve Stop-Band Attenuation. Also, a Buffer with Low Z Output or an Emitter Follower can be employed.Best

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 03, 2025, 05:31:08 am
content: HelloThanks for your answers , as the LT1001 is quite cheap ( 10 Euro on Mouser ) I will try , only problem is to locate quality Mylar cap .The CERN use low pass filter on their current pulsed calibration circuit so why not tryRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 06:42:35 am
content: The LT1001 is rather expensive and more outdated. There are now better alternatives, like ADA4077 or OPA205.The Mylar capacitors should not be that critical - the normal polyster film capacitors will work. I would consider using larger capacitors and slightly smaller resistors. Something like 2.2 µF are available and 2 in parallel is not that bad to get the 2:1 ratio. The ratio is anyway not that critical, as for a reference the exact roll-off characteristics usually does not mattter. So 1 µF and 2.2 µF or 4.7µF and 10 µF would also work.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 03, 2025, 11:39:23 am
content: Quote from: Kleinstein on April 03, 2025, 06:42:35 amHelloThanks , OPA205 are great I use them for my strain gage dynamic filter and the cost is 3 EuroPossibility to use polyester cap is great as cap supply can be tricky .RegardsOS