Noise Design Current-Feedback Circuits Kumen Blake April 199

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

Noise Design Current-Feedback Circuits Kumen Blake April 199

Datasheet Thumbnail

Top Searches for this datasheet

Noise Design Current-Feedback Circuits
Kumen Blake
April 1996
Abstract This Note covers noise model currentfeedback amps, simple design techniques useful approximations. This frequency-domain model simplify circuit analysis design. This information simplifies selection low-noise current-feedback amp. This revision obsoletes previous revision this Note, covers additional material. Contents subjects covered are:
Figure shows three input noise density sources, eni2, ibn2 ibi2, standard amplifier circuit. specifications give densities that constant over frequency (white noise). Ground inverting gain circuits, ground non-inverting gain circuits.
ibi2 OA-12 Fig1 Figure Noise Model eni2
ibn2
CLCXXX
noise model current-feedback amps Converting noise densities integrated noise Interpreting integrated noise Output noise improvement noise calculations SPICE models design example derivation Noise Power Bandwidth approximation (Appendix bibliography (Appendix
equation output voltage noise density
ibnR
ibiR (ens1 ens2
Scope Noise Analysis noise analysis this Note deals with random noise generated devices components circuit. Noise analysis gives greatest benefit when:
where:
signal level signal noise ratio (SNR) high signal sees substantial gain
Noise analysis will help:
Identify eliminate oscillation instability problems Reduce (Electro-Magnetic Interference) Reduce cross talk
voltage noise density (V/Hz) seen amp's input voltage noise density (V/Hz) amp's input current noise densities (A/Hz) 16.0 -21J 290°K temperature ens1 ens2 voltage noise densities (V/Hz) produced
Noise Model Three input-referred noise density (spot noise) sources model noise generated current-feedback (CFB) amps. Noise power density (en2 in2) power measured narrow bandwidth, normalized load resistance, units V2/Hz A2/Hz. Voltage noise density (en) current noise density (in) square-root noise power density units V/Hz A/Hz. Notice that these noise densities functions frequency.
load resistor (RL) negligible contribution noise because output resistance very small. system transfer function will shape output noise. References Appendix information generate noise transfer functions. Noise section covers excess noise (noise that exceeds white noise specifications).
1996 National Semiconductor Corporation
Printed U.S.A.
http://www.national.com
Integrated Noise Convert output voltage noise density integrated output voltage noise integrating over frequency:
Improving Output Noise reduce output noise, following: Band-limit signal after limit final output noise couple when possible low-pass filter, band-pass filter Reduce gain peaking lower NPBW Reduce resistor values lower thermal noise, keep mind that: values smaller than that recommended datasheet will cause gain peaking increased bandwidth; NPBW increase faster than intended noise reduction Smaller loads amp's output increase distortion power consumption Resistors connected input currentsensing amplifiers current noise sources; increase these resistor values reduce thermal noise
Heno
NPBW
NPBW where: Heno(jf) noise transfer function lower -3dB corner frequency ACcoupled systems, lowest frequency that affects your system's performance upper -3dB corner frequency NPBW (Noise Power Bandwidth) approximation holds when: there gain peaking NPBW approximation does hold, numerical integration instead
integrated output noise, Eno, standard deviation output noise units Vrms. also measure lower useful dynamic range. Because integrated output noise depends circuit architecture, component values amp, best compare amps based input noise densities. each noise source contributes Eno, integrate each term separately:
those amps with adjustable supply current, input noise sources change with supply current. supply current increases, input voltage noise decreases, input current noises increase, distortion improves bandwidth increases. best voltage noise performance, highest supply current. best current noise performance, lowest supply current.
Gneni
Heni
GnibnRT
Hibn
This information useful improving amplifier's SNR. Dynamic Range Signal noise ratio (SNR) describes much dynamic range signal has. compares lower useful dynamic range (Eno) signal magnitude units Vrms). input output signal noise ratios are: Vin(rms) SNRin 20log Enin Vo(rms) o(rms) SNRo 10log 20log where: Vin(rms) signal voltage input (VS1 VS2), Vrms Enin integrated voltage noise input VS2), Vrms Vo(rms) signal voltage output, Vrms integrated voltage noise output, Vrms
Noise frequencies, three input noise density terms larger than predicted specifications. dominant source this excess noise flicker) noise. Burst noise also contributes excess noise, covered this Note. input noise sources, with both noise white noise terms included, are:
fc(eni) fc(ibn) fc(ibi) where:
eni2 white noise term, eni2, c(eni) noise term, fc(eni) corner frequency noise eni2 (f); this point where eni2 doubles white noise value other input noise terms defined similarly
Notice that flicker noise power density proportional 1/f; flicker voltage noise density flicker current noise densities proportional 1/f.
http://www.national.com
integrate both white noise noise, evaluate individual noise terms separately. each term obtain:
eni2
NPBW
ibn2
CLCXXX
NPBW (1.3 NPBW1/f NPBW1/f
ibi2
OA-12 Fig2
noise contribution negligible when largest frequency that does affect your system's performance when amplifier DC-coupled. metal-film resistors minimize noise. SPICE Models SPICE models available most Comlinear's amplifiers. These models support noise simulations room temperature. recommend simulating with Comlinear's SPICE models
Figure Non-Inverting Gain Amplifier design goals are:
Provide gain non-inverting gains) DC-couple signal; lowest frequency that affects system performance 10Hz (f1) upper corner frequency 10MHz (f2) Achieve output 74dB
initial design choices made are:
Predict better value NPBW Support quicker design cycles
verify your simulations, recommend breadboarding your circuit. Evaluation boards available building testing Comlinear's amplifiers. Design Example This design example demonstrates noise design simple circuit. this example actual product; parameter values shown arbitrary illustration purposes only. This example uses non-inverting gain amplifier Figure components shown are:
20MHz pole input (this will cause reflections coax cable signal above this pole) 10MHz filter after this amplifier (not shown); this will (NPBW) 1.0k 250, recommended value, avoid gain peaking 27.8 gain
resulting junction temperature amp, input integrated noise input are: 25°C 15°C 40°C 313°K Ens1 ens1 NPBW 3.0nV/ (10.8µVrms SNRin 79.3dB does contribute output noise; nearly ideal voltage source. input source produces output noise
ens1 NPBW (108µVrms
input voltage source (with very output impedance). signal 100mVrms, voltage noise ens1 VS1) 3.0nV/Hz. coax cable placed between source amplifier match coax cable's impedance prevent reflections prevents gain peaking, filters input signal with filters input signal (this reduces signal's slew rate) gain; recommended gain noise terms are: 3.0nV/Hz fc(eni) 1.0kHz 2.0pA/Hz fc(ibn) 5.0kHz 12pA/Hz fc(ibi) 10kHz Ambient temperature 25°C Power dissipation causes 15°C junction temperature rise
13MHz
individual white noise contributions output noise are:
NPBW (108µVrms
NPBW (72µVrms NPBW (11µVrms
http://www.national.com
individual noise contributions output noise are:
NPBW1/f (10)2 3.0nV/
peaking. assume second-order transfer function circuit's high-frequency behavior: H(s)
(1kHz) 10MHz 10Hz
(3.5µVrms
NPBW1/f (1.1µVrms
NPBW1/f (5.3µVrms
contributions other components output noise are:
4kTR NPBW (10)2 4.2nV/
where natural frequency this transfer function. Integrating magnitude squared transfer function gives: NPBW
(13MHz 8Hz)
NPBW (24µVrms
(150µVrms
resulting output integrated noise, output signal output are: 227µVrms Vo(rms) GnVin(rms) 1.00Vrms SNRo 72.9dB Reduce improve SNR; this little impact other performance parameters. Changing gives: 40pF 169µVrms SNRo 75.4dB actual design, next step would SPICE simulations, then breadboarding circuit. Conclusions important points remember when designing noise circuits are:
Solving upper -3dB corner frequency (f2), substituting result equation above, gives:
NPBW
Gain peaking easy measure, strong function large easy show that: Hmax
Hmax
Employ noise analysis where small signals present Select correct resistor values reduce thermal noise Select amps based their input noise densities (integrated noise circuitdependent) Reduce NPBW gain peaking minimize integrated output noise Estimate your signal's dynamic range using Simulate with Comlinear's SPICE models estimate noise performance Build measure your circuit verify design Refer Bibliography Appendix additional background information
where Hmax peak gain magnitude. These results support following approximations: NPBW (1.3) Hmax
Hmax
with maximum error. This translates 0.8dB maximum error estimated SNR. amplifier transfer function single pole response, easy show that: NPBW single pole transfer function
Appendix (derivation Noise Power Bandwidth formula) goal estimate NPBW using common, easy measure parameters: -3dB bandwidth gain
High-order filters will have: NPBW high order filters
http://www.national.com
approximation formula includes both these cases. above results hold lower corner -3dB frequency (f1) with minor modifications. When corner -3dB frequencies interact f2), obtain: NPBW (1.3 Hmax 3.0dB
easy extend this result when there more than 3.0dB peaking, better reduce peaking, numerically integrate output noise.
Appendix (Bibliography) Motchenbacher Connelly, Low-Noise Electronic System Design, York: John Wiley Sons, 1993. Gray Meyer, Analysis Design Analog Integrated Circuits, York: John Wiley Sons, 1984. Gibson, Principles Digital Analog Communications, York: Macmillan, 1989. Carlson, Communication Systems: Introduction Signals Noise Electrical Communication, York: McGraw-Hill, 1986. Antognetti Massobrio (Editors), Semiconductor Device Modeling with SPICE, York: McGraw-Hill, 1988.
Customer Design Applications Support National Semiconductor committed design excellence. sales, literature technical support, call National Semiconductor Customer Response Group 1-800-272-9959 1-800-737-7018.
Life Support Policy National's products authorized critical components life support devices systems without express written approval president National Semiconductor Corporation. used herein: Life support devices systems devices systems which, intended surgical implant into body, support sustain life, whose failure perform, when properly used accordance with instructions provided labeling, reasonably expected result significant injury user. critical component component life support device system whose failure perform reasonably expected cause failure life support device system, affect safety effectiveness.
National Semiconductor Corporation
1111 West Bardin Road Arlington, 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018
National Semiconductor Europe
Fax: (+49) 0-180-530 E-mail: europe.support@nsc.com Deutsch Tel: (+49) 0-180-530 English Tel: (+49) 0-180-532 Francais Tel: (+49) 0-180-532 Italiano Tel: (+49) 0-180-534
National Semiconductor Hong Kong Ltd.
13th Floor, Straight Block Ocean Centre, Canton Road Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960
National Semiconductor Japan Ltd.
Tel: 81-043-299-2309 Fax: 81-043-299-2408
National does assume responsibility circuitry described, circuit patent licenses implied National reserves right time without notice change said circuitry specifications.
http://www.national.com
#350012-001

Noise Design Current-Feedback Circuits Kumen Blake April 199

Top Searches for this datasheet

Other recent searches