Block DC at Low Frequencies? - Page 1
Forum section: electronics / beginners

author: Thane of Cawdor
author_rank: Regular Contributor
timestamp: August 03, 2014, 10:39:33 am
content: Hi,I was wondering how you could block DC at ultra-low frequencies (1-10Hz) similar to an oscilloscopes AC coupling. I am trying to make a micro-vibration detector out of an analog-output 3-axis accelerometer. The problem is that there is a large DC voltage present on the signal and I only want the tiny AC spikes which represent the vibrations. How can I do this?P.S. I've tried simulating the circuit with a blocking capacitor but it seems that at low AC frequencies, the capacitor doesn't act like a short circuit.Many thanks

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 10:59:07 am
content: is the DC offset constant ? if yes, then use DC coupling and remove the offset by an op-amp.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2014, 11:09:56 am
content: If it's a digital accelerometer, do it in DSP...The reactance of a capacitance is inversely proportional to its value and the frequency, there is no "short circuit". Part of the things that "teachers lied about", like "current always takes THE shortest path", or division by zero or square roots of negative numbers.Tim

author: Thane of Cawdor
author_rank: Regular Contributor
timestamp: August 03, 2014, 02:03:23 pm
content: Thanks for the responses,QuoteThe offset is more or less constant. Would you know what the op-amp offset remover technique is called?QuoteIt's an analog-output ADXL335 accelerometer, but I couldn't feed it into a microcontroller because the vibrations that I'm trying to detect give rise to about 5mVpp (AC coupled oscilloscope) and I needed to amplify the signal this while still removing the DC offset.

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 02:12:28 pm
content: Quote from: Thane of Cawdor on August 03, 2014, 02:03:23 pmlook at this link - page4 - Offset Voltage Adjustment for Non-Inverting Amplifiers. just set the offset to the opposite value of you offset in the signal - therefore you'll nullify the offset.http://web.stanford.edu/class/ee122/Parts_Info/datasheets/op_amp_circuit%20collection_AN-31.pdf

author: w2aew
author_rank: Super Contributor
timestamp: August 03, 2014, 02:40:57 pm
content: Quote from: Thane of Cawdor on August 03, 2014, 02:03:23 pmOne name for this would be a "summing amplifier". The good news is that you can use the op amp to both functions - remove the DC offset and amplify the signal of interest. Here's a nice application note that walks you through it:http://www.ti.com/lit/an/sloa097/sloa097.pdf

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 03, 2014, 03:01:33 pm
content: Quote from: T3sl4co1l on August 03, 2014, 11:09:56 amno they wasnt. there were only people with lack of imagination.random input... 5Kohm and 10uF hi pass give you 3Hz cutoff, nice!http://www.learningaboutelectronics.com/Articles/Cut-off-frequency-calculator.php

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2014, 04:17:08 pm
content: This is pretty easy to do in the analog domain and it has the advantage over DSP of preserving the dynamic range of the ADC.Use an integrator to compare the output to whatever DC level you want, usually ground but it could also be a convenient value like 1/2 of the ADC reference voltage, and feed the integrator output back into the signal with a summing amplifier. The integrator's time constant can be adjusted with a single resistor that operates into a virtual ground which is important for the following reason; with very long integration time constants, settling time will be slow. Since the integrator's input resistor operates into a virtual ground, it is easy to shunt it with a lower value temporarily using a JFET or CMOS switch. This will speed up integration initially allowing for a fast settling time.This is one of those circuits where you probably want a low input current operational amplifier for the integrator to keep the integration capacitor size low.

author: krenzo
author_rank: Regular Contributor
timestamp: August 04, 2014, 07:46:06 pm
content: There are accelerometers available that have a vibration interrupt. When the acceleration goes over the set amount, the interrupt pin goes high until you reset it. That might be better suited to what you're trying to accomplish.