mobile | classic
Dataweek Electronics & Communications Technology Magazine





Follow us on:
Follow us on Facebook Share via Twitter Share via LinkedIn


Search...

Electronics Buyers' Guide

Electronics Manufacturing & Production Handbook 2017


 

Op-amp balancing resistors are not a given
13 September 2017, This Week's Editor's Pick, Analogue, Mixed Signal, LSI

If you grew up with the 741 op-amp, it was drilled into your head to balance resistances seen by the op-amp inputs. Over time, with different circuit techniques and different IC processes, this may not be the right thing to do. In fact, it can lead to more DC error, more noise and more instability. Why did we start doing this and what changed so that it may not be the right thing to do today?

In the 1960s and 1970s, first generation op-amps were manufactured on a plain vanilla bipolar process. To get reasonable speeds, the differential pair tail current was generally in the 10 μA to 20 μA range. Therefore, with betas of 40 to 70, the input bias current was about one microamp.

However, the transistor matching wasn't that close, so the input bias currents were not equal, resulting in a difference in the input bias currents (called input offset current) by 10% to 20% of the input bias current.

By adding a resistance (R3 in Figure 1) in the non-inverting input to ground, equal to the parallel combination of the input resistor and the feedback resistor, the impedances are made equal. Doing some algebraic manipulation, it can be shown that the error is reduced to Ioffset × Rfeedback. Because the Ioffset was 10% to 20% of Ibias, this would help in reducing the output offset error.

DC error

To reduce the input bias current on bipolar op-amps, input bias current cancellation was integrated into many op-amp designs. An example of this can be found in the OP07. With the addition of input bias current cancellation, the bias current is greatly reduced, but the input offset current can be 50% to 100% of the remaining bias current, so adding the resistor has very little effect. In some cases, adding the resistor could result in the output error actually increasing.

Noise

The thermal noise of a resistor is given by √4kTRB, so a 1 kΩ resistor will be 4 nV/√Hz. Adding a resistor will add noise. In Figure 2, surprisingly, even though the 909 Ω compensation resistor is the lowest value because of noise gain from that node to the output, it contributes the most noise at the Figure 2 output. Output noise due to R1 is 40 nV/√Hz, for R2, 12.6 nV/√Hz, and for R3, 42 nV/√Hz. So don't use a resistor.

On the other hand, if the op-amp is powered from split supplies and one supply comes up before the other one, there may be latch-up problems with the ESD network, in which case it may be desirable to add some resistance to protect the part. But if used, a bypass cap should be placed across the resistor to reduce the noise contribution of the resistor.

Figure 1. Classic inverting amplifier.
Figure 1. Classic inverting amplifier.

Figure 2. Noise analysis.
Figure 2. Noise analysis.

Figure 3. What you see.
Figure 3. What you see.

Figure 4. What the electrons see.
Figure 4. What the electrons see.

Stability

All op-amps have some input capacitance, both differential and common mode. If the op-amp is connected as a follower and the impedances are balanced by putting a resistor in the feedback path, the system may become prone to oscillation. The reason is that with a large feedback resistor, the input capacitance of the op-amp, and the stray capacitance on the PC board, an RC low-pass filter (LPF) is formed. This filter causes phase shift and will reduce the phase margin of the closed-loop system. If it reduces it too much, the op-amp will oscillate.

A customer was using an AD8628 CMOS op-amp in a 1 Hz, Sallen-Key low-pass filter circuit. Because of the low corner frequency, the resistors and capacitors were rather large (see Figure 3). The input resistor was 470 kΩ, so the customer put a 470 kΩ in the feedback. This resistor, in combination with the eight picofarads of input capacitance (see Figure 4), gave the customer a pole at 42 kHz.

The AD8628 has a gain bandwidth product of 2 MHz, so it still had plenty of gain at 42 kHz and it oscillated rail-to-rail. Changing the 470 kΩ resistor to a 0 Ω jumper solved the problem. So avoid large resistors in the feedback where large depends on the gain bandwidth of the op-amp. For high frequency op-amps, such as the ADA4817-1 with a gain bandwidth over 400 MHz, a 1 kΩ feedback would be large. Always read the data sheet for recommendations.

Summary

Over the years, rules of thumb are developed that serve a purpose. On a design review, it's always good practice to look carefully at these rules and see if they are still applicable. With respect to adding a balancing resistor, if the op-amp is CMOS, JFET or bipolar with input bias current cancellation, you probably don't need one.

For more information contact Conrad Coetzee, Arrow Altech Distribution, +27 (0)11 923 9600, ccoetzee@arrow.altech.co.za, www.arrow.altech.co.za


Credit(s)
Supplied By: Arrow Altech Distribution (AAD)
Tel: +27 11 923 9600
Fax: +27 11 923 9884
Email: info@arrow.altech.co.za
www: www.arrow.altech.co.za
  Follow us on Facebook Share via Twitter Share via LinkedIn    

Further reading:

  • What are my IoT data transport options?
    15 November 2017, Otto Wireless, This Week's Editor's Pick, Telecoms, Datacoms, Wireless
    There are three main factors to consider when deploying a wireless IoT sensor or controller: power, range (network coverage) and speed.
  • Addressing safety concerns with Li-ion batteries
    15 November 2017, Uniross Batteries, This Week's Editor's Pick, Power Electronics / Power Management
    Lithium-ion is safe but with millions of consumers using batteries, failures are bound to happen.
  • Production cycle of a sealed lead-acid battery
    15 November 2017, Forbatt SA, This Week's Editor's Pick, Power Electronics / Power Management
    It’s easy when commissioning or simply plugging a battery into the device it is destined for, to overlook all the steps that went into its production. Often the minerals that are used in making a battery ...
  • NFC/RFID Nucleo expansion board
    15 November 2017, Arrow Altech Distribution (AAD), Telecoms, Datacoms, Wireless
    The X-NUCLEO-NFC04A1 from STMicroelectronics helps developers easily embed in their designs a dual-interface ST25DV tag IC with both NFC/RFID contactless interface and a wired I²C connection.    Offering ...
  • It’s not about the board
    15 November 2017, This Week's Editor's Pick, Computer/Embedded Technology, Design Automation
    It might sound controversial, but choosing the best board for a project shouldn’t start with a comparison of different boards and appraising the features and benefits. With such a wealth of development ...
  • AAD supports Dell OEM offering
    15 November 2017, Arrow Altech Distribution (AAD), Computer/Embedded Technology
    Arrow Altech Distribution (AAD) is offering the full range of Dell EMC OEM products and solutions to customers so that IoT (Internet of Things) solutions can be designed and delivered to the market place. ...
  • Home-grown prototyping board
    15 November 2017, Arrow Altech Distribution (AAD), Electronics Technology
    Arrow Altech Distribution created new options for electronics makers by producing the locally made Cherry Blossom, based on the AM335x reference design from Texas Instruments and similar to BeagleBone ...
  • Non-isolated switching regulators
    15 November 2017, Arrow Altech Distribution (AAD), Power Electronics / Power Management
    Murata has added a 12 V model to its OKI-78SR series of non-isolated switching regulator DC-DC power converters for embedded applications. The fixed single-output converter offers both tight regulation ...
  • EMI feedthrough filter
    15 November 2017, Arrow Altech Distribution (AAD), Circuit & System Protection
    NexTek announced the addition of a new EMI feedthrough filter model to its existing range of filter solutions. As the company’s first standard multistage EMI filter, this new 180 A design offers unique ...
  • Dimmable LED driver
    15 November 2017, Arrow Altech Distribution (AAD), Opto-Electronics, Power Electronics / Power Management
    The AL1673 from Diodes Incorporated is a high-performance, single-stage LED driver-converter operating in flyback and buck-boost topologies, targeting dimmable LED lighting applications. It operates in ...
  • Wi-Fi/Bluetooth combo module
    15 November 2017, Arrow Altech Distribution (AAD), Telecoms, Datacoms, Wireless
    Sierra Wireless announced the BX Series of Wi-Fi and Bluetooth combo modules with built-in cloud services and security features. Housed on a single common flexible form factor (CF3) module, the product ...
  • Three-phase half-bridge gate driver
    15 November 2017, Arrow Altech Distribution (AAD), Power Electronics / Power Management
    Diodes Incorporated recently announced the DGD2136, a fully integrated three-phase gate driver IC intended for driving N-channel MOSFETs or IGBTs in a half-bridge configuration. With its floating high-side ...

 
 
         
Contact:
Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronic Buyers Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Other
Terms & conditions of use, including privacy policy
PAIA Manual





 

         
    Classic | Mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.