In this age of packetized video transmission, Analog Devices Inc.'s (ADI) crosspoint switches and diff-amps reinforce the notion that not all signals are best handled in the digital domain. Though we've come a long way since the days of the mechanical crosspoint switch, there remain myriad solid-state systems that can benefit from smooth switching of analog signals under digital control. Typical systems accommodate video as well as audio.
The high bandwidth ICs that switch these signals need be fast and linear (to avoid distortion), yet ideally they would dissipate low power. They also need to be able to handle RF and video signal levels that typically range from millivolt levels up to a volt or two in amplitude. It's also essential that inter-channel crosstalk be held to a minimum as well.
ADI's AD8175 and AD8176 triple 16 x 9 crosspoint switches look like they shape up to meet these criteria. When teamed up with ADI's AD814x differential amplifiers, you should be able to design a video switching system with comparative ease.
Nine 16:1 RBG MUXs
Let's look a bit closer at ADI's switches. In operation, an AD8175, for example, comprises a non-blocking crosspoint with 16 differential RGB (red-green-blue) input channels and nine differential RGB output channels. Internally, the device is organized into nine 16:1 RBG multiplexers, with each MUX responsible for connecting an RGB input channel to its respective RGB output channel.
Decoding logic selects a single input (or none) in each MUX, connecting it to its respective output. Feedback around each MUX provides a closed-loop differential-in/differential-out gain of 2. A second loop provides a closed-loop common-mode-in gain of unity.
Each differential RGB input channel is also buffered by a differential receiver. These receivers are capable of accepting input common-mode voltages extending to either supply rail. In addition to passing differential signals, each receiver can process and route input common-mode voltages.
Responding To Common-Mode Voltages
Significantly, the IC's receivers let you drive the inputs with an uncertain common-mode voltage, such as from a remote source over twisted pair. The receivers respond only to the differences in input voltages, restoring common-mode levels internally that are suitable for the downstream internal signal path. As a common-mode system, noise and/or crosstalk, which affect each receiver's inputs equally, are rejected by the IC's input stage.
The overall common-mode voltage of all three differential pairs comprising an RGB channel is processed and rejected by a separate circuit block. So, a static discharge or a resistive voltage drop in a middle-of-CAT5-run with sync-on common-mode signaling coupling into all three pairs in an RGB channel would be rejected at the AD8175's output, while sync-on common-mode signals would be permitted through the switch.
Serial Or Parallel Programming
Either serial or parallel TTL-compatible programming of the chip's switches is also possible. All outputs are simultaneously updated.
In serial mode, a serial output pin permits multiple AD8175 or AD8176 chips to be daisy-chained together for single-pin programming. In these cases, a SEROUT (serial output) signal from one device can be connected to the SERIN of the next device to form the serial chain.
In a parallel programming mode—where it isn't required that you re-program the entire device when making changes to the device's matrix—you can modify a single output or more at a time. This takes only one WE/UPDATE (Write Enable) cycle, making parallel programming faster than serial programming.
Power Source Flexibility
The AD8175 also imparts a good degree of power source flexibility. It can be powered by either a ±2.5-V source or a single-ended +5-V supply. Split-supply operation with ±2.5-V lines lets you readily interface to ground-referenced video signals.
Additional logic lets control logic supplies (VDD/DGND) run off 5-V/0-V to +3.3-V/0-V while the IC's analog core remains on split supplies.
Additional flexibility in analog output common-mode level and output black level lets you use unequally split power supplies. If, for example, +3-V/-2-V supplies to +2-V/-3-V supplies are used, the output common-mode can still be set to zero volts for ground-referenced video signals. Finally, the device's pins are ESD (electro-static discharge) protected.
Typical dissipation for this rather large 676-pin BGA (ball grid array) package is about 3.5-W. However, because these switches can drive low-impedance loads down to 100 ohms, dissipation can vary a great deal depending on what is switching.
Indeed, ADI cautions users that excessive dissipation and resulting out-of-bound temperatures can actually distort the internal die, altering performance, or worse, resulting in catastrophic destruction. Fortunately, the AD8175's outputs can be disabled to minimize on-chip dissipation, but caution is needed in both board layout and design to ensure safe operation.
Horizontal And Vertical Sync
Although the 500-MHz device is intended for switching differential-in/differential-out middle-of-CAT5-run applications, each differential RGB output channel is complemented by horizontal and vertical syncs for use in end-of-CAT5-run single-ended-output applications, where a video monitor is typically driven.
These chips also a support sync-on common-mode operating mode, and a sync-on color operating mode. A device's outputs can also be placed in a high-impedance Disable state that supports connection of multiple devices with minimal loading on an output bus. There's also a Reset pin that can disable all outputs.
With independent output buffers, one of these ICs can also be placed into a high-Z state to create larger arrays. This is done by paralleling crosspoint outputs. Inputs can be paralleled, too.
As you can gather from ADI's press statement (on the left), output stages are also fast slew-rate types that exhibit fast settling times when driving series-terminated CAT5 cables. Significantly, small-signal bandwidth approaches large-signal bandwidth.
The Diff-Amps
For their parts in a video switching matrix, the companion AD8145, AD8146, AD8147, and AD8148 diff-amps comprise differential-to-single-ended receivers. Apart from garden variety diff-amps, these devices were specifically tailored by ADI for receiving RGB video over twisted pairs, or across differential circuit board traces.
Yes, these ICs can be used as general-purpose comparators, but thanks to their wide bandwidth specs they can receive virtually any type of analog signal, or even high-speed data, and can decode video sync signals encoded on received common-mode voltages. With their high common-mode rejection (69-dB), they support the use of unshielded twisted pair cabling, as do the associated switches.
Using so-called active feedback (unlike that of most op-amps), these amplifiers provide separate pairs of differential inputs. One of these input pairs is usually driven by a differential input signal, and the other is for feedback.
Active Feedback Benefits
The active feedback scheme offers a number of benefits. For one, you sustain high common-mode rejection, as well as accommodating a wide input common-mode range. An external feedback network establishes the gain response, but its separate path makes it independent of signal input. This helps to cut interaction between the feedback and the input circuits.
Another advantage of ADI's active feedback architecture is the ability to change the polarity of the gain by switching the device's differential inputs; thus a high input-Z inverting amplifier can be established. Also, a unity-gain inverter produces low output-noise.
When you consider the combination of ADI's analog crosspoint switches and the specialized amplifiers that support them, the combo adds up to a straightforward way to transmit and switch high-resolution content. That can be just the ticket for distributing video to remote displays.
Although some specs are still to be determined, take a gander at these preliminary datasheets for more information. ADI's Web site appears to indicate that all-important evaluation boards will eventually be available, too.
Click here for an AD8175 preliminary datasheet (in Adobe Acrobat .PDF format).
Click here for an AD8145 preliminary datasheet (in Adobe Acrobat .PDF format).
For further details, contact Analog Devices, Inc., 804 Woburn St., Wilmington, Mass. 01887. Phone: 800-262-5643. Fax: 781/937-1021. E-mail: customer.service@analog.com
Analog Devices, 800-262-5643, www.analog.com
