Calibration of temperature sensors – I am working with two different temperature sensors. One of them is a themistor and the other is an RTD. I put them in the same place and the temperature that they both measured differed in 0.1 degrees C. However, when the temperature rises to 50 degrees then to 80 degrees, the difference in the temperature widens to a 10 degrees of difference. How do I make the two sensors more accurate and stable and have a matched reading? Read more
Voltage follower with op amp – I have two microcontrollers talking to each other (two Laughpad kits) through SPI, but today I realized that one is powered with 3.3V and the other one is powered with 3.6V, which means that one’s high logic level is 3.6V. After reading the datasheet of the 3.3V microcontroller, it says that the maximum voltage applied to any pin is Vcc + 0.3V, which would be exactly 3.6V. I think that this is too risky. To solve this problem, I was thinking of using a voltage follower op amp to limit the 3.6V high logic level to 3.3V. If I power the op amp with 3.3V, can I input 3.6V on it without damaging it? Read more
Choosing the ratio L and C for an LC tank oscillator – When designing a traditional LC tank oscillator whose oscillation frequency is given by 1/√LC, how can I chose the ratio L and C? I have no area restriction and I can design the inductor with any size and shape. Read more
Troubleshooting ultrasonic distance detection – I’m troubleshooting a new design and don’t understand some of the results. Perhaps someone has some insight?
This is the circuit:
A short burst (3-4 cycles) of 40kHz is applied to the transducer via U4, the mux is then switched to listen mode.
The transducer rings for about 600uS after the burst completes (as expected and seems reasonable). With nothing for the burst to reflect off, I would expect the receive signal to remain quiet after the initial decay, however a signal is detected about 1.4ms after the initial pulse, I have no idea why this is happening. Read more
Removing 400 mHz envelope signal – We are testing a very sensitive magnetic field sensor for few weeks and we are experiencing a ~400mHz (like 40 Min period) wave which is acting as an envelope. We don’t know where it is coming from. because our system does not use such a frequency any where. Please let me know if you have any idea to remove it. Read more
LED driver LM317 schematic – I need to build a variable driver for an LED lamp using the LM317. The lamp can operate from 2.7v @ <10mA up to 3.1v @ 400mA for full brightness. Can you suggest a suitable adjustable voltage driver using the LM317? Read more
Switched cap low pass filter in audio circuit – I’m working on an audio circuit with a frequency response of 600 – 3000Hz. The input signal has significant high frequency noise (sounds like 10kHz white noise). An 8th order SCF (MAX7400) with a cutoff of 3kHz has little effect audio wise. However, if I inject a signal and measure the attenuation of the filter the signal is down by 50dB before 4kHz at the output of the filter. So the filter is working, however the background noise (when I listen with an audio amplifier) seems about the same at the input and output of the filter. The tone of the nose changes slightly but that’s about it. Looking at the signal with a scope there is significant noise at the SCF oscillator frequency of 300kHz (10mVp-p), and some even higher frequency, lower level noise ~0.5mV at perhaps 1MHz but nothing obvious at lower frequencies. So I can’t see what I hear. Any suggestions? Read more
LPF for removal of radio frequency transmission – How do you determine the value of resistor and capacitor for an LPF that would remove radio frequency transmission? This type of filter is required for metal detection. Read more
Input current of op amp – I recently used the TL072 op amp in my design. This op amp is a J-FET input and according to its datasheet and its input current should be in range of pA. However, I measure 1 to 8 mA current in both + and – pins of the IC. How can this be possible? Read more
Op amp AC simulation – Usually I use a large resistor (>1M ohm) to break the loop between output/ inverting input (feedback path) of op-amp. A large capacitor also adds (1F) at inverting input. AC + DC level source added at non-inverting input to run simulation. What is the difference I put AC + DC source at inverting (between big capacitor and ground) or non-inverting input. Also, what is the consideration to set capacitor and resistor value and how do I decide the optimized value? Read more
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