FMS6502 datasheet(10/15 Pages) ONSEMI

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FMS6502 Rev. 1.0.0

Applications Information

Input Clamp / Bias Circuitry

The FMS6502 can accommodate AC- or DC-coupled

inputs. Internal clamping and bias circuitry are provided

to support AC-coupled inputs. These are selectable

through the CLMP bits via the I2C-compatible interface.

For DC-coupled inputs, the device should be pro-

grammed to use the 'bias' input configuration. In this con-

figuration, the input is internally biased to 625mV through

a 100k

Ω resistor. Distortion is optimized with the output

levels set between 250mV above ground and 500mV

below the power supply.

With AC-coupled inputs, the FMS6502 uses a simple

clamp rather than a full DC-restore circuit. For video sig-

nals with and without sync; (Y,CV,R,G,B), the lowest volt-

age at the output pins is clamped to approximately

300mV above ground.

symmetric

AC-coupled

input

signals

are

used

(Chroma,Pb,Pr,Cb,Cr), the bias circuit can be used to

center them within the input common range. The aver-

age DC value at the output is approximately 1.27V.

Figure 8 shows the clamp mode input circuit and the

internally controlled voltage at the input pin for AC-cou-

pled inputs.

Figure 8. Clamp Mode Input Circuit

Figure 9 shows the bias mode input circuit and the inter-

nally controlled voltage at the input pin for AC-coupled

inputs.

Figure 9. Bias Mode Input Circuit

Output Configuration

The FMS6502 outputs may be AC or DC-coupled. DC-

coupled loads can drive a 150

Ω load. AC-coupled out-

puts are capable of driving a single, doubly terminated

video load of 150

Ω. An external transistor is needed to

drive DC low-impedance loads.

DC-coupled outputs

should be connected as indicated in Figure 10.

Figure 10. DC-Coupled Load Connection

Configure AC-coupled loads as shown in Figure 11.

Figure 11. AC-Coupled Load Connection

When an output channel is not connected to an input, the

input to that particular channel’s amplifier is forced to

approximately 150mV. The output amplifier is still active

unless specifically disabled by the I2C interface. Voltage

output levels depend on the programmed gain for that

channel.

Driving Capacitive Loads

When driving capacitive loads, use a 10

Ω-series resis-

tance to buffer the output, as indicated in Figure 12.

Figure 12. Driving Capacitive Loads

FMS6502

Input

Clamp

0.1µF

Video source must

be AC coupled

Lowest voltage

set to 125mV

FMS6502

Input

Bias

0.1µF

Video source must

be AC coupled

Average voltage

set to 625mV

FMS6502

Output

Amplifier

FMS6502

Output

Amplifier

220µF

FMS6502

Output

Amplifier

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