CMV1020
CALIFORNIA MICRO DEVICES
perfected by TV technicians in the 1950’s. A gimmick
is made by taking two lengths (typically about a foot)
of small gauge wire such as AWG 24, twisting them
together, and then after baring all ends soldering the
gimmick across RF. With the circuit operating, CF is
"adjusted" by clipping short lengths of the gimmick off
until the compensation is nominal. Then simply
remove the gimmick, take it to an impedance bridge,
and select the capacitor accordingly.
6.Typical Applications
Operational amplifiers have been used for years to
generate frequency stable oscillators, but the circuit
shown in Figure 4 provides a stable frequency operat-
ing from a single supply voltage and drawing a mere
40µA. For (R1 + R2) ÷ R1 = 0.473, the period, T, of the
oscillator is given by:
T = 2 RF C1
Where: RF is the feedback resistor
C1 is the capacitor
The period is easily adjusted by varying RF. R3
ensures that the circuit will start on a single rail by
forcing A1’s output to the positive rail. R4’s value is not
critical but should be a factor of 10 greater than the
parallel combination of R1 and R2. The circuit lends
itself to a variety of applications such as battery
operated toys where a stable frequency is required and
low supply current is a must to maintain battery life.
4. Capacitive Load Considerations
The CMV1020 is capable of driving capacative loads
in excess of 100pF without oscillation. However,
significant peaking will result. Probably the easiest
way minimize this problem is to use an isolation
resistor as shown in Figure 3.
Figure 4
Personal Computers including laptops are available
with sophisticated and high quality audio capabilities.
Battery conservation is a key issue with laptop
computers, and the circuit in Figure 5 utilizes the low
supply current of the CMV1020, its rail to rail output
voltage swing, and its high output current drive to
provide the interface to the microphone input. A1 is
used to provide the common mode bias for A2 by
buffering the VREF output (typically 2.2 volts) of the
Codec and to supply bias to the microphone. R1 should
be selected for the appropriate bias for the micro-
phone. R3 and C2 provide low pass filtering for noise,
and the closed loop gain of A2 is adjusted by the ratio
of R5 to R4.
Figure 3
5. Power Supply Decoupling
The CMV1020 is not prone to oscillation without the
use of power supply decoupling capacitors, however to
minimize hum and noise pick-up, it is recommended
that the rails be bypassed with 0.01µF capacitors.
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Tel: (408) 263-3214
Fax: (408) 263-7846
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