XR8038 Datasheet, PDF (7/16 Pages) Exar Corporation – Precision Waveform Generator

XR-8038A
V_CC
11
R_A
R₂
I_A
10K
78
Buffer ₄
10
V_CC ^{SWITCH S}
R₁
40K
C
R_B
Buffer ₅
2I_B
11
V_EE
Figure 3. Detailed View of Current Sources I_A and 2I_B.
WAVEFORM ADJUSTMENT
The symmetry of all waveforms can be adjusted with the
external timing resistors. Two possible ways to
accomplish this are shown in Figure 4, Figure 5, and
Figure 6. Best results are obtained by keeping the timing
resistors R_A and R_B separate (Figure 4.) R_A controls the
rising portion of the triangle and sine wave and the “low”
state of the square wave.
The magnitude of the triangle waveform is set at 1/3 V_CC;
therefore, the duration of the rising proportion of the
triangle is:
pins 4 and 5 can be shorted together, as shown in
Figure 6. This connection, however, carries an inherently
larger variation of the duty cycle.
With two separate timing resistors the frequency is given
by:
ǒ Ǔ ^f
+
1
t₁ ) t₂
+
1
5
3
·R
A^C
1
)
RB
2RA–RB
or, if R_A = R_B = R
t₁
+
C·|DV|
I_A
+
C·|
2
3
V_CC-
VCC
1
3
V_CC|
+
5
3
R_A·C
5RA
The duration of the falling portion of the triangle and sine
wave and the ”low” state of the square wave is:
t₂
+
C·|DV|
2I_B-I_A
+
C^·|
2
3
V
CC^-
1
3
V
CC^|
- ^{2VCC VCC}
5RB 5RA
+
5
3
·
R_AR_BC
2R_A-R_B
Thus a 50% duty cycle is achieved when R_A = R_B
If the duty-cycle is to be varied over a small range about
50%, the connection shown in Figure 5 is slightly more
convenient. If no adjustment of the duty cycle is desired,
f
+
0.3
RC
(for Figure 4. )
If a single timing resistor is used (Figure 5 and Figure 6),
the frequency is:
f
+
0.15
RC
The frequency of oscillation is independent of supply
voltage, even though none of the voltages are regulated
inside the integrated circuit. This is due to the fact that
both currents and thresholds are direct, linear function of
the supply voltage and thus their effects cancel.
Rev. 2.01
7