AD8178 Datasheet, PDF (33/40 Pages) Analog Devices – 450 MHz, Triple 16 × 5 Video Crosspoint Switch

AD8178
V_POS
QNPN
QPNP
I_{O, QUIESCENT}
V_OUTPUT
I_{O, QUIESCENT}
I_OUTPUT
V_NEG
Figure 51. Simplified Output Stage
Example
With an ambient temperature of 85°C, all nine RGB output
channels driving 1 Vrms into 100 Ω loads, and power supplies at
±2.5 V, follow these steps:
1. Calculate the power dissipation of the AD8178 using data
sheet quiescent currents, neglecting the VDD current because
it is insignificant.
PD,QUIESCENT = (VPOS × IVPOS) + (VNEG × IVNEG)
(4)
PD,QUIESCENT = (2.5 V × 460 mA) + (2.5 V × 460 mA) = 2.3 W
2. Calculate power dissipation from loads. For a differential
output and ground-referenced load, the output power is
symmetrical in each output phase.
PD,OUTPUT = (VPOS − V ) OUTPUT,RMS × IOUTPUT,RMS
(5)
PD,OUTPUT = (2.5 V − 1 V) × (1 V/100 Ω) = 15 mW
There are 15 output pairs, or 30 output currents.
nPD,OUTPUT = 30 × 15 mW = 0.45 W
3. Subtract quiescent output stage current for the number of
loads (30 in this example). The output stage is either standing
or driving a load, but the current needs to be counted only
once (valid for output voltages > 0.5 V).
PDQ,OUTPUT = (VPOS − VNEG) × IOUTPUT,QUIESCENT
(6)
PDQ,OUTPUT = (2.5 V − (−2.5 V)) × 1.65 mA = 8.25 mW
There are 15 output pairs, or 30 output currents.
nPD,OUTPUT = 30 × 8.25 mW = 0.25 W
4. Verify that the power dissipation does not exceed the
maximum allowed value.
P = P + nP − nP D,ON-CHIP
D,QUIESCENT
D,OUTPUT
DQ,OUTPUT
(7)
PD,ON-CHIP = 2.3 W + 0.45 W − 0.25 W = 2.5 W
From Figure 50 or Equation 1, this power dissipation is below
the maximum allowed dissipation for all ambient temperatures
up to and including 85°C.
In a general case, the power delivered by the digital supply and
dissipated into the digital output devices has to be taken into
account following a similar derivation. However, because the
loads driven by the H and V outputs are high and because the
voltage at these outputs typically sits close to either rail, the
correction to the on-chip power estimate is small. Furthermore,
the H and V outputs are active only briefly during sync
generation and returned to digital ground thereafter.
Short-Circuit Output Conditions
Although there is short-circuit current protection on the AD8178
outputs, the output current can reach values of 80 mA into
a grounded output. Any sustained operation with too many
shorted outputs can exceed the maximum die temperature
and can result in device failure (see the Absolute Maximum
Ratings section).
Crosstalk
Many systems (such as KVM switches) that handle numerous
analog signal channels have strict requirements for keeping the
various signals from influencing any of the other signals in the
system. Crosstalk is the term used to describe the coupling of
the signals of other nearby channels to a given channel.
When there are many signals in close proximity in a system,
as is undoubtedly the case in a system that uses the AD8178,
the crosstalk issues can be quite complex. A good understanding
of the nature of crosstalk and some definition of terms is required
to specify a system that uses one or more crosspoint devices.
Types of Crosstalk
Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field, and the
sharing of common impedances. This section explains these effects.
Every conductor can be both a radiator of electric fields and
a receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter propagates
across a stray capacitance (for example, free space) and couples
with the receiver and induces a voltage. This voltage is an unwanted
crosstalk signal in any channel that receives it.
Currents flowing in conductors create magnetic fields that circulate
around the currents. These magnetic fields then generate voltages
in any other conductors whose paths they link. The undesired
induced voltages in these other channels are crosstalk signals.
The channels that crosstalk can be said to have a mutual inductance
that couples signals from one channel to another.
The power supplies, grounds, and other signal return paths
of a multichannel system are generally shared by the various
channels. When a current from one channel flows in one of these
paths, a voltage that is developed across the impedance becomes
an input crosstalk signal for other channels that share the common
impedance.
All these sources of crosstalk are vector quantities, so the magni-
tudes cannot simply be added together to obtain the total crosstalk.
In fact, there are conditions where driving additional circuits in
parallel in a given configuration can actually reduce the crosstalk.
The fact that the AD8178 is a fully differential design means that
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