Hi I'm newbie interested in audio electronics
I practice a simple basic non inverting opamp amplifer with input capacitor for prevent input dc offset with LT spice.
I designed 3db frequency of the high pass filter is about 10hz
(Its just not exact value but I think it is ok to block input dc offset)
This amp works very well but FFT seems very high noise floor at low frequency.
with input capacitor of 2.2uF/ 10kohm
the noise floor is 85db at 20Hz, 140db at 10kHz
without input capacitor
under 140db at audible frequency with plotwinsize 0, numdgt 7 options.
I know the capacitor makes thermal noise like resistor, I guess the impedance of capacitor is getting bigger at lower frequency so the input capacitor induced noise probably be bigger at low frequency.
However I think There are So May GOOD amps with 2.2uf input capactor that does NOT have these kind of noise floor degradation at low frequency problems.
I Want to solve these kind of problems. Please help me.
P.S I don't want to use DC servo, output capacitors.
I practice a simple basic non inverting opamp amplifer with input capacitor for prevent input dc offset with LT spice.
I designed 3db frequency of the high pass filter is about 10hz
(Its just not exact value but I think it is ok to block input dc offset)
This amp works very well but FFT seems very high noise floor at low frequency.
with input capacitor of 2.2uF/ 10kohm
the noise floor is 85db at 20Hz, 140db at 10kHz
without input capacitor
under 140db at audible frequency with plotwinsize 0, numdgt 7 options.
I know the capacitor makes thermal noise like resistor, I guess the impedance of capacitor is getting bigger at lower frequency so the input capacitor induced noise probably be bigger at low frequency.
However I think There are So May GOOD amps with 2.2uf input capactor that does NOT have these kind of noise floor degradation at low frequency problems.
I Want to solve these kind of problems. Please help me.
P.S I don't want to use DC servo, output capacitors.
You could do worse than have a mosey through this thread :http://www.diyaudio.com/forums/tubes-valves/105496-how-make-sense-ltspice-fft-analysis.html
most Spice device models don't give device noise in .TRAN - which is what you need for FFT
unless you have added Behavioral Source noise generators to your models
so you are almost certainly looking at settling time and windowing
try running the sim for 5-10x longer, just fft the last 4-5 periods (with Blackman window) of the test signals
unless you have added Behavioral Source noise generators to your models
so you are almost certainly looking at settling time and windowing
try running the sim for 5-10x longer, just fft the last 4-5 periods (with Blackman window) of the test signals
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
- Home
- Design & Build
- Software Tools
- [LTSPICE] Why input-capacitor degrades noise floor?