FMS3818

DATA SHEET

2

REV. 1.2.3 December 2004

Functional Description

Within the FMS3818 are three identical 8-bit D/A

converters, each with a current source output. External loads

are required to convert these currents to voltage outputs.

green video signals.

Figure 1. FMS3818 Current Source Structure

Digital Inputs

Incoming GBR data is registered on the rising edge of the

clock input, CLK. Analog outputs follow the rising edge of

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

connected to the green D/A converter. SYNC = H adds a

112/256 fraction of full-scale current to the green output.

SYNC = L extinguishes the sync current during the sync tip.

Figure 2. Nominal Output Levels

BLANK gates the D/A inputs. If BLANK = H, the D/A

inputs control the output currents to be added to the output

blanking level. If BLANK = L, data inputs and the pedestal

are disabled.

D/A Outputs

Expressed in current units, the GBR transformation from

data to current is as

follows:

Typical LSB current step is 73.2 µA.

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 resis-

To implement a doubly-terminated 75

Ω transmission line, a

shunt 75

Ω resistor should be placed adjacent to the analog

output pin. With a terminated 75

Ω line connected to the

analog output, the load on the FMS3818 current source is

37.5

Ω.

The FMS3818 may also be operated with a single 75 Ohm

terminating resistor. To lower the output voltage swing to the

be doubled.

Voltage Reference

which can be derived from either the 1.25 volt internal

bandgap reference or an external voltage reference

To minimize noise, a 0.1µF capacitor should be connected

Power and Ground

Required power is a single +3.3 Volt supply. To minimize

power supply induced noise, analog +3.3V should be

pin pair.

High slew-rate 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.

SYNC

data: 700 mV max.

sync: 307 mV