FMS6502 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – 8-Input, 6-Output Video Switch Matrix with Output Drivers, Input Clamp, and Bias Circuitry

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 100kΩ 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.
If 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.
Lowest voltage
set to 125mV
0.1µF
Video source must
be AC coupled
75
FMS6502
Input
Clamp
Output Configuration
The FMS6502 outputs may be AC or DC-coupled. DC-
coupled loads can drive a 150Ω load. AC-coupled out-
puts are capable of driving a single, doubly terminated
video load of 150Ω. An external transistor is needed to
drive DC low-impedance loads. DC-coupled outputs
should be connected as indicated in Figure 10.
FMS6502
Output
Amplifier
75
75
Figure 10. DC-Coupled Load Connection
Configure AC-coupled loads as shown in Figure 11.
FMS6502
Output
Amplifier
75 ^220µF
75
Figure 11. AC-Coupled Load Connection
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.
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.
Average voltage
set to 625mV
0.1µF
Video source must
be AC coupled
75
FMS6502
Input
Bias
Driving Capacitive Loads
When driving capacitive loads, use a 10Ω-series resis-
tance to buffer the output, as indicated in Figure 12.
FMS6502
10
Output
Amplifier
C_L
Figure 9. Bias Mode Input Circuit
© 2006 Fairchild Semiconductor Corporation
FMS6502 Rev. 1.0.0
Figure 12. Driving Capacitive Loads
www.fairchildsemi.com
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