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VFC110APG4 Datasheet(PDF) 8 Page - Texas Instruments |
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VFC110APG4 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 12 page VFC110 SBVS021A − OCTOBER 1988 − REVISED APRIL 2007 www.ti.com 8 Using the Enable input, several VFCs’ outputs can be connected to a single output line. All disabled VFCs will have a high output impedance; one active VFC can then transmit on the output line. Since the disabled VFCs are not oscillating, they cannot interfere or lock with the operating VFC. Locking can occur when one VFC operates at nearly the same frequency as—or a multiple of—a nearby VFC. Coupling between the two may cause them to lock to the same or exact multiple frequency. It then takes a small incremental input voltage change to unlock them. Locking cannot occur when unneeded VFCs are disabled. REFERENCE VOLTAGE The VREF output is useful for offsetting the transfer function and exciting sensors. Figure 3 shows VREF used to offset the transfer function of the VFC110 to achieve a bipolar input voltage range. Sub-surface zener reference circuitry is used for low noise and excellent temperature drift. Output current is specified to 10mA and current-limited to approximately 20mA. Excessive or variable loads on VREF can decrease frequency stability due to internal heating. 2 NC V IN 112 11 One−Shot V REF 4 13 5V 3 6 8 f OUT 7 5 R PU C OS R 1 14 NC +5V + 15V 10 − 15V R 2 I REF (∼1mA) Figure 3. Offsetting the Frequency Output PRINCIPLE OF OPERATION The VFC110 uses a charge-balance technique to achieve high accuracy. The heart of this technique is an analog integrator formed by the integrator op amp, feedback capacitor CINT, and input resistor RIN. The integrator’s output voltage is proportional to the charge stored in CINT. An input voltage develops an input current of VIN/RIN, which is forced to flow through CINT. This current charges CINT, causing the integrator output voltage to ramp negatively. When the output of the integrator ramps to 0V, the comparator trips, triggering the one-shot. This connects the reference current, IREF, (approximately 1mA) to the integrator input during the one-shot period, TOS. This switched current causes the integrator output to ramp positively until the one-shot period ends. Then the cycle starts again. The oscillation is regulated by the balance of current (or charge) between the input current and the time-averaged reset current. The equation of current balance is: where TO is the one-shot period and fOUT is the oscillation frequency. I IN + I IREF Duty Cycle V IN R IN + I REF f OUT T O Integrator Output (Pin 12) 0V f OUT 1/f OUT T OS Effect of Smaller C INT |
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