TMC3503

PRODUCT SPECIFICATION

2

REV. 1.02 11/24/99

Functional Description

The TMC3503 is a low-cost triple 8-bit CMOS D/A con-

verter designed to directly drive computer CRT displays at

pixel rates up to 80 Msps. It comprises three identical 8-bit

D/A converters with registered data inputs, common clock,

and internal voltage reference. An independent current

source allows sync to be added to any D/A converter output.

Digital Inputs

All digital inputs are TTL-compatible. Data are registered on

the rising edge of the CLK signal. The analog output

of pipeline delay on the chip. The guaranteed clock rates of

the TMC3503 are 80, 50, and 30 MHz.

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 turns off a separate current

Figure 1. Nominal Output Levels

pendently. It is commonly tied to the green D/A converter for

“Sync on Green” operation. This connection adds a 40 IRE

sync pulse to the D/A output and brings that D/A output to

0.0 Volts during the sync tip. SYNC and BLANK are regis-

tered on the rising edge of CLK.

BLANK gates the D/A inputs and sets the pedestal voltage.

If BLANK = HIGH, the D/A inputs are added to a pedestal

which offsets the current output. If BLANK = LOW, data

inputs and the pedestal are disabled.

WHITE

The WHITE control drives all three D/As to full-scale, over-

riding the data inputs. It is overridden by the BLANK input,

and is independent of SYNC.

data: 660 mV max.

65-3503-02

pedestal: 54 mV

sync: 286 mV

SLEEP

The SLEEP control, when HIGH, places the TMC3503 in a

power-down state. This function operates asynchronously.

D/A Outputs

Each D/A output is a current source. To obtain a voltage out-

put, 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

Normally, a source termination resistor of 75 Ohms is con-

nected between the D/A current output pin and GND near

the D/A converter. A 75 Ohm coaxial cable may then be con-

nected 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 TMC3503 may also be operated with a single 75 Ohm

terminating resistor. To lower the output voltage swing to the

increased.

Voltage Reference

The TMC3503 has an internal bandgap voltage reference of

+1.235 Volts. An external voltage reference may be con-

ence. All three D/A converters are driven from the same

reference.

A 0.1µF capacitor must be connected between the COMP

low-noise operation.

Power and Ground

The TMC3503 D/A converter requires a single +5.0 Volt

should be decoupled to GND to reduce power supply

induced noise. 0.1µF decoupling capacitors should be placed

as close as possible to the power pins.

The 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 distor-

tion or reduced signal-to-noise performance. All ground pins

should be connected to a common solid ground plane for

best performance.