designideas
0.02 F FILM 1.62k 1.62k
+
0.02 F FILM VDC ADJUST 1 F FILM DC BLOCK OP AMP AND ADC UNDER TEST 3.3V 10k
+
15V OUTPUT AMPLITUDE ADJUST 2k TRIMPOT –15V 1.24k
CHRISTMAS LAMP C7 120V 5W
LT1468-2
−
3.3V 324 1% 470 pF NPO 6.65k 1% 22 pF VREF CS LTC2361 SDO SCK GND 0VDD VDD
1k
LTC6255
−
AIN
100
10k 1%
HP 3325 ADC CLOCK GENERATOR
Figure 2 The Wien-bridge oscillator drives the op amp and ADC pair under test. The resulting FFT is clean after windowing, but not exceptional, as Figure 3 shows. Figure 3 This 4096-point FFT was achieved using an unlocked oscillator with a 92-dB Blackman-Harris window. Note that the peak does not look like –1 dBFS and that there is power in the bins around the peak.
OSCILLATOR INCOHERENT, CLEANED UP WITH 92-dB BLACKMAN HARRIS WINDOW 0 DOESN’T LOOK LIKE −1 dBFS –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 0 512
EDN DI5291 Fig 2.eps DIANE
POWER IN BINS OUTSIDE THE PEAK
VIN=–1 dBFS, 5 kHz FS=125 kSPS SFDR=89 dB
NOT AN ACCURATE PICTURE OF THE HARMONICS
1024
1536
2048
environment? At 5 kHz, a passive filter with notches would be large and fussy. I thought of detuning the Wien-bridge oscillator by reducing the gain, thereby converting it into a filter. But then it occurred to me that a gentle, analog sinusoidal nudge from a distorting but well-locked external oscillator might be enough to
tweak the Wien-bridge frequency to where it needed to be. I decided to try injecting a sinusoid into the input of the Wien-bridge op-amp circuit, and opted to use a high series impedance to avoid simultaneously injecting noise and distortion. I came up with 200k—about 1000× the impedances already there—and put it in as shown on
EDN DI5291 Fig 3.eps
the left side of Figure 4 (the “new input”). I set up the Agilent 33250A for a 5-kHz sine wave and applied it to the new input. Looking at both the 33250A and the Wien-bridge outputs with an oscilloscope, I slowly dialed up the 33250A frequency and was thrilled to finally see the sinusoids come “close” and then snap into lock.
DIANE
38 EDN EuropE | december 2012
www.edn-europe.com
http://www.edn-europe.com
Table of Contents for the Digital Edition of EDNE December 2012