Changing voltages, industry protocol, and dc isolation needs can dictate when and how the EE needs to ac couple
BY DUANE SORLIE
Fairchild Semiconductor
South Portland, ME
http://www.fairchildsemi.com
Today’s integrated video filter/driver devices can be designed into video systems (set-top boxes, PVRs, TVs) using a wide range of coupling and clamping configurations, such as input ac- or dc-coupling, output ac- or dc-coupling, and various input clamping and bias configurations. With all of these configurations, designers have a number of decisions to make.
When the designer decides to ac-couple an input, he or she needs to understand that the dc content of the incoming video signal becomes lost, so the dc bias level will have to be set by the filter/driver’s input bias and clamp circuitry. If the filter input does not have a clamp and bias circuit or an active dc restore loop, then an external bias network will need to be added in order to set the input common-mode level correctly.
Ac coupling
Ac coupling the analog video input signal into a given device is very common in video and graphics systems to allow the receiving device to set its own optimal dc bias level (on the device side of the capacitor) independent of the driving signal’s dc bias level. For example, the receiving device of an analog-to-digital converter (ADC) may set the clamping or blanking levels of the video signal equal to the internal ADC code zero voltage, regardless of the driving signal’s absolute dc level.
Three video filter circuits featuring an onboard clamp circuit configured for input and output ac coupling (A); a configuration operating in the SAG mode (B); and a direct-couple method (C).
Another example would be a pure analog system where the receiving device may wish to set the analog signal’s common-mode level around VCC/2 to optimize its signal-processing headroom. The receiving device can also match the “clamped” level to a predetermined dc reference voltage, allowing for a consistent and stable dc output voltage. By blocking the dc component, the receiving device protects itself from any potentially damaging dc current flow.
Selecting capacitors
Let’s look at how to select the correct capacitor for coupling the video input signal to the video filter/driver device in Section A of the figure. In order to limit the low-frequency droop (tilt) associated with ac coupling, the location of the lower 3-dB cutoff frequency should be set properly.
In this case, the bandwidth of the video signal requires a large enough capacitor to pass the minimum frequency, which is the 50- or 60-Hz frame rate. The input circuit consists of the ac-coupling capacitor and the input impedance of the video filter device.
To determine the lower 3-dB corner frequency, the designer would use the formula f=1/2πRC. Using a 0.1-µF capacitor and the 800 KΩ input impedance, the value of 2Hz is calculated, which is more than adequate to pass the 50 or 60 Hz frame rate.
Since capacitors short at higher frequencies, high-frequency rolloff is not a concern. In most applications a good tantalum 0.1-µF coupling capacitor with very low ESR will do the job adequately.
After selecting the correct input-coupling capacitor, the next step is to determine the value of the output coupling capacitor. Assume in this case that the device needs to drive a transmission line that is back-terminated with 75 Ω, so the output capacitor is working into the effective resistance of 150 Ω.
Since the load is a relatively low-impedance transmission line—and because we still need to pass the 50- or 60-Hz frame rate—the output coupling capacitor will need to be relatively large. Using the same calculation as we did for the input coupling capacitor—this time using a 220-µF coupling capacitor and the 150-Ω load—we calculate the corner frequency to be 4.8 Hz, which again is adequate to pass the frame rate.
Most applications have more stringent field tilt requirements and use a 470 or 1,000-µF device as the coupling capacitor. Ac-coupling the output requires the receiving device to set the common-mode level on its input, independent of the incoming video signal’s dc level.
The 75-Ω series termination resistor should be placed as close to the filter/driver device output as possible. This helps isolate downstream parasitic capacitance and inductance from the output of the device and provides for optimal signal conditions.
SAG mode
Some disadvantages of ac-coupling the output are the need for a physically large and expensive capacitor, signal tilt or droop (unless a very large coupling capacitor is used), and loss of the video signal dc component. If ac-coupling is required, one might consider a video filter device that incorporates a SAG function, illustrated in Section B of the figure.
The SAG function is a feedback network incorporated into the device circuitry that eliminates the need for very large coupling capacitors. Using a SAG function allows the use of much smaller capacitors that have capacitance values about 10 times lower than typical ac-coupled configurations.
A video filter device configured in the SAG mode will typically display a dc gain of 9 dB (3x), and return to its normal 6 dB (2x) gain at about 25 Hz. This poses a problem when the device is implemented in a mobile device using a 3-V power source because the offset at the output of the device (before the load) sits at around 750 mV.
The typical video input is 1 V p-p. The filter/driver device has a gain of 2x, which puts the output at 2 Vp-p plus the dc offset of 750 mV, which now puts the high side of the signal at 2.75 V.
This condition will drive the signal into the VCC rail at the low supply voltage of 2.7 V and start to clip the top of the video signal causing unwanted signal distortion. The offset can be overcome by placing a resistor between the SAG pin and VCC , which will lower the dc offset into a range that prevents the output signal from being clipped over the normal supply voltage range.
Alternative method
An alternative to ac coupling video signals is to incorporate the direct couple method as shown in Section C of the figure. There are many devices available in the market that can be used in system designs for either ac- or dc-coupled applications.
The intent of dc-coupling is to drive these devices by an input that is single-ended and ground-referenced. An example would be a standard current-mode output of a video/graphics DAC.
These common DAC devices use the doubly terminated 75-Ω load (37.5 Ω) as the load for the current-steering DAC to develop the output voltage. Therefore, the DAC output in this type of system has a known dc level that is ground-referenced.
The video filter/driver devices in these families are intended to work seamlessly with a video DAC output and provide the following advantages:
No need for an input coupling capacitor
No potential settling time for the clamp
No tilt from the input cap discharge
No input impedance limitation
No need for the on-chip sync stripper, charge pump circuitry, and servo loop
Dc coupling the output is the most direct approach used to feed a video signal to a visual media device. This eliminates the need for a coupling capacitor and allows for a tilt-free signal to be sent to the media device.
Some of the disadvantages of this approach are that the receiving device needs to know the incoming dc levels so it can process the video signal properly, and there is no feedback control on the absolute dc level of the output voltage, which may vary with system temperature and supply voltage. Most media devices are designed using ac coupling at their input and then using dc restoration of the signal at the ADC for proper color control.
Knowing this, dc-coupling the output signal seems like the most cost-effective straightforward approach. Dc-coupling also eliminates the occurrence of the output signal being doubly ac-coupled: once at the output of the filter/driver device then again at the input of the media device. ■
For more information on coupling capacitors, visit www.electronicproducts.com/passives.asp
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