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MC1408-8DG Datasheet(PDF) 5 Page - ON Semiconductor |
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MC1408-8DG Datasheet(HTML) 5 Page - ON Semiconductor |
5 / 9 page MC1408−8 http://onsemi.com 5 Circuit Description The MC1408−8 consists of a reference current amplifier, an R−2R ladder, and 8 high−speed current switches. For many applications, only a reference resistor and reference voltage need be added. The switches are non−inverting in operation; therefore, a high state on the input turns on the specified output current component. The switch uses current steering for high speed, and a termination amplifier consisting of an active load gain stage with unity gain feedback. The termination amplifier holds the parasitic capacitance of the ladder at a constant voltage during switching, and provides a low impedance termination of equal voltage for all legs of the ladder. The R−2R ladder divides the reference amplifier current into binary−related components, which are fed to the remainder current which is equal to the least significant bit. This current is shunted to ground, and the maximum output current is 255/256 of the reference amplifier current, or 1.992 mA for a 2.0 mA reference amplifier current if the NPN current source pair is perfectly matched. Functional Description Reference Amplifier Drive and Compensation The reference amplifier input current must always flow into Pin 14. regardless of the setup method or reference supply voltage polarity. Connections for a positive reference voltage are shown in Figure 3. The reference voltage source supplies the full reference current. For bipolar reference signals, as in the multiplying mode, R15 can be tied to a negative voltage corresponding to the minimum input level. R15 may be eliminated and Pin 15 grounded, with only a small sacrifice in accuracy and temperature drift. The compensation capacitor value must be increased with increasing values of R14 to maintain proper phase margin. For R14 values of 1.0, 2.5, and 5.0 k W, minimum capacitor values are 15, 37, and 75 pF. The capacitor may be tied to either VEE or ground, but using VEE increases negative supply rejection. (Fluctuations in the negative supply have more effect on accuracy than do any changes in the positive supply.) A negative reference voltage may be used if R14 is grounded and the reference voltage is applied to R15, as shown in Figure 4. A high input impedance is the main advantage of this method. The negative reference voltage must be at least 3.0 V above the VEE supply. Bipolar input signals may be handled by connecting R14 to a positive reference voltage equal to the peak positive input level at Pin 15. Capacitive bypass to ground is recommended when a DC reference voltage is used. The 5.0 V logic supply is not recommended as a reference voltage, but if a well regulated 5.0 V supply which drives logic is to be used as the reference, R14 should be formed of two series resistors and the junction of the two resistors bypassed with 0.1 mF to ground. For reference voltages greater than 5.0 V, a clamp diode is recommended between Pin 14 and ground. If Pin 14 is driven by a high impedance such as a transistor current source, none of the above compensation methods apply and the amplifier must be heavily compensated, decreasing the overall bandwidth. A1 A2 A3 A4 A5 A6 A7 A8 5 6 7 8 9 10 11 12 3 MC1408 13 14 15 1 2 4 16 SEE TEXT FOR VALUES OF C. C R14 = R15 (+)VREF R14 R15 RL IO VEE VCC Figure 3. Positive VREF A1 A2 A3 A4 A5 A6 A7 A8 5 6 7 8 9 10 11 12 3 MC1408 13 14 15 1 2 4 16 SEE TEXT FOR VALUES OF C. C R14 = R15 (−)VREF R14 R15 RL IO VEE VCC Figure 4. Negative VREF |
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