Voltage References

Digitally Modulating an Analog Signal
The MC10324 may be used to digitally modulate (or
attenuate) an analog signal by applying the analog signal to
the reference amplifier. Three methods of doing this are
shown in Figures 13 to 15.
Figure 13. Applying an Analog Signal Directly
Ra = R,e!
1-1.7V < V,e! < - O.8V]
Ra
Figure 14. Capacitor Coupling the AC Voltage
Ra = R,e!
1-1.7V < V,e! < - O.8V]
Figure 15. Applying a Modulating Current
Ra = R,e!
1-1.7V < V,e! < - O.8V]
Figure 16 shows a typical video application circuit
using the MC10324 in the clocked mode. The clock is
single-ended, and the circuit updates the output on the
rising edge of the clock. The Out - pin feeds a standard
75 n monitor through a 75 n cable, which is terminated
at both ends. The reference voltage is supplied by an
LM385-1.2 regulator.
MC10324
In all three examples the DC reference current is Vref/Rref.
In Figure 13 the AC signal source is referenced to a negative
voltage source (Vref). In Figures 13 and 14 the AC reference
current is equal to VAC divided by Rref. In Figure 15 the AC
reference current is equal to VAC divided by Rb. The AC
signal at Out - and Out + is determined by the following
equations:
VO-(AC)
Va + (AC)
TYPICAL APPLICATION CIRCUITS
Iref(AC) x (255-A) x RL
16
Iref(AC) x A x RL
where "A" is the value of the digital word at DO - D7 (0 to 255).
When implementing any of the above schemes, or any
other method of feeding an AC signal to the reference
amplifier, the following operating limits must be observed:
1) The peak values of the reference current
(AC + DC) must be within the range of 0.5 rnA
to 1.7 rnA into Pin 15;
2) The peak values of the voltage at Ref + and Ref -
must be within the range of - 0.8 V to -1.7 V;
3) The peak values of the voltage at Out - and Out +
must be within the range of -1.7 V to + 0.3 V.
The maximum frequency which can be handled by the
reference amplifier is dependent on the compensation
capacitor (CC) at Pin 16, and the signal amplitude according
to the following equation:
fmax=
MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA
6-89
16
1.59 x 10- 8
Cc x Ipk
where Ipk is the peak value of the AC reference current (1/2 of
the peak-to-peak value). The small signal bandwidth of the
reference amplifier is = 3.0 MHz.
Components associated with the reference amplifier (Pins
14 - 16) should be physically close to the pins. The board
layout should be neat, preventing unwanted stray capacitive
coupling between the outputs and the reference amplifier. If
Cc is smaller than 5000 pF a ground plane is strongly
recommended. Cc should not be smaller than 250 pF.
Figure 17 shows a circuit similar to that of Figure 16,
except the MC10324 is used in the transparent mode. The
source of the data bits must provide the 8-bits
simultaneously, with minimum skew, to keep output glitches
to a minimum. If latches, or other anti skew circuitry, are not
available within the microprocessor circuitry, a set of 8-bit
latches between it and the MCI 0324 is recommended, or the
MC10324 should be used in the clocked mode.