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| 型号： | AN013 |
| 厂商： | Intersil Corporation |
| 英文描述： | JT 128C 128#22D SKT RECP |
| 中文描述： | 人们始终想关于ICL8038知情权 |
| 文件页数： | 2/4页 |
| 文件大小： | 87K |
| 代理商： | AN013 |
2
Question 5
I have a similar duty cycle problem when I use high values of
R
A
and R
B
. 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 Q
9
and into the external C. The
discharge current is the current in R
B
which flows down
through diode Q
8
. Adding to the Q
8
current is the current of
Q
7
which is only a few microamperes. Normally, this Q
7
cur-
rent is negligible, but with a small current in R
B
, 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 V
CC
+2V) where V
CC
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 R
A
and R
B
rather than one resistor with pins 4 and 5 connected
together. This is because transistors Q
2
and Q
3
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 Q
5
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 R
B
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 R
B
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 (10M
to 20M
) 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
You’re probably having trouble keeping the constant voltage
across R
A
and R
B
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 (0.1
μ
F 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/
o
C to maintain
overall low frequency drift at V
CC
= 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
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