AN013

AN013 INTERSIL | Alldatasheet

Document overview

• Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
• PDF pages: 4

Technical content

the care and feeding of the 8038 to customers worldwide. and breadboard in hand, read on and be enlightened. from 1V to 0.1V will also change the frequency by 10:1. tors which make this large sweep range less than ideal. these resistors, pin 8 must be raised above V+. course, limit the frequency sweep range to around 100:1. FIGURE 1. VARIABLE AUDIO OSCILLATOR, 20Hz TO 20kHz

I have a similar duty cycle problem when I use high values of R A and RB. What causes this? Answer There is another error term which becomes important at very low charge and discharge currents. This error current is the emitter current of Q 7. The application note on the 8038 gives a complete circuit description, but it is sufficient to know that the current charging the capacitor is the current in R A which flows down through diode Q9 and into the external C. The discharge current is the current in RB which flows down through diode Q8. Adding to the Q8 current is the current of Q 7 which is only a few microamperes. Normally, this Q7 cur- rent is negligible, but with a small current in RB, this current will cause a faster discharge than would be expected. This problem will also appear in sweep circuits when the voltage across the external resistors is small. Question 6 How can I get the lowest distortion over the largest frequency sweep range. Answer First of all, use the largest supply voltage available (±15V or +30V is convenient). This will minimize V BE mismatch prob- lems and allow a wide variation of voltage on pin 8. The potential on pin 8 may be swept from V CC (and slightly higher) to 2/3 VCC +2V) where VCC is the total voltage across the 8038. Specifically for±15V supplies (+30V), the voltage across the external resistors can be varied from 0V to nearly 8V before clipping of the triangle waveform occurs. Second, keep the maximum currents relatively large (1mA or 2mA) to minimize the error due to Q 7. Higher currents could be used, but the small geometry transistors used in the 8038 could give problems due to V CE(SAT) and bulk resistance, etc. Third, and this is important, use two separate resistors for RA and RB rather than one resistor with pins 4 and 5 connected together. This is because transistors Q2 and Q3 form a differ- ential amplifier whose gain is determined by the impedance between pins 4 and 5 as well as the quiescent current. There are a number of implications in the differential amplifier con- nection (pins 4 and 5 shorted). The most obvious is that the gain determines the way the currents split between Q 2 and Q 3. Therefore, any small offset or differential voltage will cause a marked imbalance in the charge and discharge cur- rents and hence the duty cycle. A more subtle result of this connection is the effective capacitance at pin 10. With pins 4 and 5 connected together, the “Miller Effect” as well as the compound transistor connection of Q 3 and Q5 can produce several hundred picofarads at pin 10, seriously limiting the highest frequency of oscillation. The effective capacitance would have to be considered important in determining what value of external C would result in a particular frequency of oscillation. The single resistor connection is fine for very sim- ple circuits, but where performance is critical, the two sepa- rate resistors for R A and RB are recommended. Finally, trimming the various pins for lowest distortion deserves some attention. With pins 7 and 8 connected together and the pot at pin 7 and 8 externally set at its maxi- mum, adjust the ratio of R A and RB for 50% duty cycle. Then adjust a pot on pin 12 or both pins 1 and 12 depending on minimum distortion desired. After these trims have been made, set the voltage on pin 8 for the lowest frequency of interest. The principle error here is due to the excess current of Q 7 causing a shift in the duty cycle. This can be partially compensated for by bleeding a small current away from pin 5. The simplest way to do this is to connect a high value of resistance (10MΩ to 20MΩ ) from pin 5 to V- to bring the duty cycle back to 50%. This should result in a reasonable com- promise between low distortion and large sweep range. Question 7 This waveform generator is a piece of junk. The triangle wave is non-linear and has large glitches when it changes slope. Answer Y ou’re probably having trouble keeping the constant voltage across R A and RB really constant. The pulse output on pin 9 puts a moderate load on both supplies as it switches current on and off. Changes in the supply reflect as variations in charging current, hence non-linearity. Decoupling both power supply pins to ground right at the device pins is a good idea. Also, pins 7 and 8 are susceptible to picking up switching transients (this is especially true on printed circuit boards where pins 8 and 9 run side by side). Therefore, a capacitor (0.1µF or more) from V+ to pin 8 is often advisable. In the case when the pulse output is not required, leave pin 9 open to be sure of minimizing transients. Question 8 What is the best supply voltage to use for lowest frequency drift with temperature? Answer The 8038AM, 8038AC, 8038BM and 8038BC are all temper- ature drift tested at V CC = +20V (or±10V). A curve in the lower right hand corner of Page 4 of the data sheet indicates frequency versus temperature at other supply voltages. It is important to connect pins 7 and 8 together. Question 9 Why does connecting pin 7 to pin 8 give the best temperature performance? Answer There is a small temperature drift of the comparator thresh- olds in the 8038. To compensate for this, the voltage divider at pin 7 uses thin film resistors plus diffused resistors. The different temperature coefficients of these resistors causes the voltage at pins 7 and 8 to vary 0.5mV/ oC to maintain overall low frequency drift at VCC = 20V. At higher supply voltages, e.g.,±15V (+30V), the threshold drifts are smaller compared with the total supply voltage. In this case, an externally applied constant voltage at pin 8 will give reason- ably low frequency drift with temperature. Application Note 013