Top 10 analog threads on EDAboard.com – August

Crystal load capacitance – In the book, Experimental Methods in RF Design”, the authors state that “Our crystal had marked frequency of 7045 KHz. This was the specified frequency for operation with a 32-pF load capacitance.” If I had a 20MHz crystal, how will I know the exact value of its load capacitance? Read more

Selecting proper values for 20 Khz instrumentation amplifier – I’ve designed an instrumentation amplifier to receive a 20KHz signal from another coil at the Tx side such that this would give ZERO output if there is NO METAL between the TX and RX. And this would give an output voltage if there is any metal in between TX and RX.  Would you please guide me to select the proper values of resistor for that? Read more

Simulating CMOS ring oscillator in AWR – We are trying to simulate a CMOS ring oscillator using AWR AO using 180 nm technology and the maximum VDD allowed is 1.8V  But the problem is that we are getting oscillations only at approximately 5V bias. No matter how much we change the widths and lengths, the oscillations occur only at a bias voltage of 5V. We are using BSIM3 version for the NMOS and PMOS. We understand that the problem is with the MOSFET model used. We tried the AWR example of push push oscillator and the output was obtained beautifully at the desired voltage. Please help us in understanding what parameters or model of the MOSFET has to be changed so as to obtain an output at the desired voltage. Read more

How to choose values in class C amplifier design – Unfortunately none of the websites I have found on Class C amplifier design speak about how to choose the values of C1 and R1 (look at the circuit below).

I think that C1 & R1 form a HPF, if the input signal is of 2V peak and 10MHz frequency. I think that if you choose to value of C1 such that it has very low reactance at 10MHz, then using the high-pass RC filter law to calculate R1 when cutoff freq = 10MHz or lower. Is this true?  Read more

Triggering cd14538 – I’ve been doing some tests with a circuit based around a CD14538. In the datasheet it says it can be triggered by either positive going edge or negative going edge. In this situation, I need it to be triggered by the second option (negative). During breadboarding, I noticed that the IC triggered with positive edges despite the fact that it was configured the other way. Could it be because of the wires’ inductance is causing some sort of oscillation? I decided to put a low pass at the trigger input to smooth the signal a little and it worked, but I wanted to ask if anyone had some experience with this IC and faced the same situation. Read more

Problem with amplifier for current transducer – I am trying to make a current transducer with full differential output. It will be used for data acquisition. The transducer uses a isolation amplifier optocoupler (U1) and a full differential instrumentation amplifier with an integrated ADC driver (IC1).
Everything is OK to the isolation amplifier. When I put 8 A on the input I get the correct output on the TP2 and TP3. But when I connect the isolation amplifier output (U1) to the input of the differential amplifier (IC1), the output TP2 and TP3 goes down to the half of the value. I tried to put a 1M Ω resistor on the inputs of the amplifier and even so the output of the opto drops to the half. As I know the current on the input of the amplifier was supposed to be insignificant (correct?) why could this be happening? Need some help to understand whats going on. The voltage supplies are okay and the board is okay. Read more

Measuring motor RPM using magnetic sensor with LM324 – I need to measure the RPM of a motor. I have a magnetic sensor, which is interfaced with an LM324 with output to an ATMega88PA. I need an exact pulse signal. I have designed a circuit, but this circuit does not produce constant pulses. Read more

Is LT1360 a low-noise op amp? – I want a verify does the LT1360 is a low noise amplifier because it was not mentioned in the data sheet. I’m confused when comparing it with others device. For instance, in the  AD8055 data sheet, it mentions that the IC is low noise ( 6 nV/√Hz ), but at a frequency of 100kHz while the LT1360 has 9nV/√Hz at 10kHz frequency. Could anybody explain? Read more

Fuse resistor calculation – Fuse resistors are rated by “fuse power”. How does that translate to the real world when you need to protect a PSU by limiting inrush current? So far, my assumption is (10uF)  Rlim = Vpeak/Iinrush = 240*sqrt(2)/38A = 10R. But how do you compute the required power of resistor? Read more

Resonator driver circuit – circuit or simulator? – In this circuit simulation below, I am using LTSpice. I am getting the current through the inductor as sinusoidal. But, the voltage across the inductor is not sinusoidal in nature. Is it a problem of the circuit or problem of the simulator? Read more