In this multipart series:

Bandwidth selection
After choosing an amplifier and the associated resistors and capacitors, the next step is to design for best bandwidth (BW). Be careful not to over-design for a wide bandwidth. The bandwidth should be wide enough to pass the fundamental frequencies and important harmonics, but no wider. Select an amplifier that has enough BW and follow it by an RC filter. The amplifier itself is also a single pole filter. Amplifiers and resistors have noise over each Hertz of BW so the greater the BW, the greater the output noise and the lower the SNR.

Figure 6 shows the effects of the amplifier's BW vs. noise for the same circuit configuration as before, but using amplifiers with different BW. To limit the added noise, the BW should be narrowed as much as possible.

Figure 6: Output voltage noise increases with increasing amplifier bandwidth


(Click on image to enlarge)

To narrow the bandwidth, use an RC filter after the sensor. This can create loading problems that can be overcome by using a buffer as shown in Figure 7 .

Figure 7: Add a buffer between the sensor and filter to avoid loading problems.
(Click on image to enlarge)

An amplifier and ADC having the specs and configuration shown (amplifier BW 350 MHz) will have 166 μVrms noise. Adding an RC filter after the op amp, creating an effective BW of 50 MHz, reduces the noise to 56 μVrms.

Narrowing the BW by using the correct RC will greatly improve the SNR as shown, but the resistor itself can add noise. A better way to reduce the BW is by using the circuit shown in Part C of Figure 8 . This places the resistor inside the op amp's feedback loop, reducing its effect by 1 + loop gain. Don't forget to reduce power supply noise from the signal path by using adequate decoupling caps at the supply pins.

Figure 8: a) simple RC filter; b) buffer reduces loading; c) putting resistor inside feedback loop minimizes noise.


(Click on image to enlarge)

After going through these steps, review the other system requirements. Below are some examples:

• Do the selected components meet the other target specs?
• Does the amplifier require dual supplies?

Is there a positive supply?

• Does the amplifier consume too much power?
• Are the components too expensive?

If necessary, return to Step 1 and repeat the process.

Conclusion
Every sensor has its own noise, impedance and response characteristics, so matching these to the analog front-end is critical. A well defined low-noise design process is needed to overcome many challenges in today's applications and to get the best SNR. This iterative process will produce a signal conditioning solution that is most suitable for today's challenging applications.

References
(note: all from Analog Devices, Inc, except the book)

1. Online noise seminars on this topic listed at: http://www.analog.com/en/content/0,2886,759%255F786%255F109826%255F0,00.html#seminars
2. Application Note AN-202, An IC Amplifier User's Guide to Decoupling, Grounding, and Making Things Go Right for a Change ,
   http://www.analog.com/static/imported-files/application_notes/135208865AN-202.pdf
3. Application Note AN-358, Noise and Operational Amplifier Circuits , http://www.analog.com/static/imported-files/application_notes/5480117281535838576388017880AN358.pdf
4. Application Note AN-940, Low Noise Amplifier Selection Guide for Optimal Noise Performance , http://www.analog.com/static/imported-files/application_notes/AN_940.pdf
5. Bryant, James Bryant and Lew Counts, Op Amp Issues — Noise , Analog Dialogue, Volume 24 Issue 2, 1990
6. Motchenbacher, C. D., and J. A. Connelly, Low-Noise Electronic System Design . New York: John Wiley & Sons, Inc.1993

About the author
Reza Moghimi is an applications engineering manager for the Precision Analog Products group at Analog Devices, Inc. He holds a BSEE and MBA from San Jose State University (SJSU), CA. In addition to Analog Devices, Reza has worked for Raytheon Corp., Siliconix Inc, and Precision Monolithic Inc. (PMI). He enjoys traveling, music and soccer.