FMS3810/3815 PRODUCT SPECIFICATION
2
REV. 1.08 12/21/00
Functional Description
Within the FMS3810/3815 are three identical 10-bit D/A
converters, each with a current source output. External loads
are required to convert the current to voltage outputs. Data
inputs RGB
7-0
are overridden by the BLANK input. SYNC
= H activates, sync current from I
OS
for sync-on-green video
signals.
Digital Inputs
All digital inputs are TTL-compatible. Data is registered on
the rising edge of the CLK signal. Following one stage of
pipeline delay, the analog output changes t
DO
after the rising
edge of CLK.
SYNC
and BLANK
SYNC and BLANK inputs control the output level (Figure 1
and Table 1) of the D/A converters during CRT retrace
intervals. BLANK forces the D/A outputs to the blanking
level while SYNC = L turns off a current source that is
connected to the green D/A converter. SYNC = H adds a 40
IRE sync pulse to the green output, SYNC = L sets the green
output to 0.0 Volts during the sync tip. SYNC and BLANK
are registered on the rising edge of CLK.
Figure 1. Nominal Output Levels
BLANK gates the D/A inputs and sets the pedestal voltage.
If BLANK = H, the D/A inputs are added to a pedestal which
offsets the current output. If BLANK = L, data inputs and the
pedestal are disabled.
D/A Outputs
Each D/A output is a current source. To obtain a voltage
output, a resistor must be connected to ground. Output
voltage depends upon this external resistor, the reference
voltage, and the value of the gain-setting resistor connected
between R
REF
and GND.
Normally, a source termination resistor of 75 Ohms is
connected between the D/A current output pin and GND
near the D/A converter. A 75 Ohm line may then be
connected with another 75 Ohm termination resistor at the
far end of the cable. This “double termination” presents the
D/A converter with a net resistive load of 37.5 Ohms.
The FMS3810/3815 may also be operated with a single 75
Ohm terminating resistor. To lower the output voltage swing
to the desired range, the nominal value of the resistor on
R
REF
should be doubled.
Voltage Reference
All three D/A converters are supplied with a common
voltage reference. Internal bandgap voltage reference voltage
is +1.235 Volts with a 3K
Ω
source resistance. An external
voltage reference may be connected to the V
REF
pin,
overriding the internal voltage reference.
A 0.1µF capacitor must be connected between the COMP
pin and V
DD
to stabilize internal bias circuitry and ensure
low-noise operation.
Power and Ground
Required power is a single +5.0 Volt supply. To minimize
power supply induced noise, analog +5V should be
connected to V
DD
pins with 0.1 and 0.01 µF decoupling
capacitors placed adjacent to each V
DD
pin or pin pair.
High slew-rate of digital data makes capacitive coupling to
the outputs of any D/A converter a potential problem. Since
the digital signals contain high-frequency components of the
CLK signal, as well as the video output signal, the resulting
data feedthrough often looks like harmonic distortion or
reduced signal-to-noise performance. All ground pins should
be connected to a common solid ground plane for best
performance.
data: 660 mV max.
pedestal: 54 mV
sync: 286 mV
