A technical discussion on analog signal processing circuits and solutions
• Mar 26, 2013

Paralleling Op Amps—is it possible?

Is it possible to parallel two op amps to get twice the output current? We get this question periodically on our E2E forums. Though we may answer with a qualified “yes,” it tends to make us shudder just a bit. It can be done… but with great care. So let me come quickly to a key point. Don’t use the simple circuit on the left...
• Mar 19, 2013

Resistor Puzzle Solution... and a rant on schematics

Did you see last week’s resistor puzzle? Check it out if you missed it . Here’s the solution: We’re not accustomed to reading three-dimensional schematics so the first step is to redraw it clearly. There are three distinct paths from A to B, colored blue, green and red. Each has a series connection of 1Ω—R—1Ω...
• Mar 12, 2013

Resistor Puzzle—the sequel

It’s time for some fun! I’ve known a few folks who have tormented colleagues with a resistor cube—equal resistors on all sides. So in case you’ve solved that one, let’s add a twist. In this cube, not all the resistors are equal. The resistance from A to B is 1Ω. Resistor values are indicated, except for those marked...
• Mar 3, 2013

1/f Noise—the flickering candle

The 1/f (one-over-f) low frequency noise region of amplifiers seems just a bit mysterious. Reader “tweet” asked for a discussion of 1/f noise—a challenging topic for a short blog. Click Here to read on EDN magazine web site. Excel noise calculation file here… (Please visit the site to view this file) Note: This file...
• Feb 26, 2013

Simulating Gain-Bandwidth—the generic op amp model

It may not always be obvious how the gain-bandwidth product (GBW) of an op amp may affect your circuits. Macro-models have a fixed GBW. Though you can look inside these models, it’s best not to tinker with them. What to do? You can use a generic op amp model in SPICE to check your circuits for sensitivity to GBW. Most SPICE-based circuit simulators...
• Feb 19, 2013

ESD… Zapp!

We’ve included device-level ESD performance of our ICs in data sheets for many years. But these figures apply to an integrated circuit before soldering onto your circuit board. What about ESD tolerance on your PCB ? We qualify the ESD performance by zapping each pin multiple times on several devices. It simulates nasty mistreatment that might...
• Feb 10, 2013

Op Amp Noise—but what about the feedback?

Last month we explored noise of the non-inverting amplifier but I dodged the issue of the feedback network’s noise contribution. A reader, Jim, challenged me—he wanted more detail. So what about the noise from the feedback network? Click Here to read on EDN magazine web site. Excel noise calculation file here…(Please visit the...
• Jan 28, 2013

Comparators—what’s all the chatter?

It’s an easy concept—the inputs compare two voltages. The output is high or low. So, why all the chatter through the transition? This effect usually occurs with slow changes through the transition voltage. Often it’s because the input signals have noise that jiggles through the transition voltage causing a chattering output. Even...
• Jan 22, 2013

Put Gain Up Front—waxing philosophical

Low noise, low offset voltage, low drift—all the precision low-level signal processing goals get easier when you put voltage gain up front in the signal chain. It’s a simple concept. The error in the second stage is divided by the gain of the first stage, figure 1. For example, with only a modest first-stage gain of ten, the second stage...
• Jan 15, 2013

Input Capacitance—common-mode?...differential?… huh?

The input capacitance specifications of op amps are often confused or ignored. Let’s clarify how these specs can best be used. Stability of an op amp circuit can be affected by input capacitance at the inverting input by causing phase shift&mda...