AD8145(Rev0) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8145 Datasheet PDF : 24 Pages
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APPLICATIONS
OVERVIEW
The AD8145 contains three independent active feedback amplifiers
that can be effectively applied as differential line receivers for
red-green-blue (RGB) signals or component video signals, such
as YPbPr, transmitted over unshielded twisted pair (UTP) cable.
The AD8145 also contains two general-purpose comparators
with hysteresis that can be used to receive digital signals or to
extract video synchronization pulses from received common-
mode signals that contain encoded synchronization signals.
The comparators, which receive power from the positive
supply, are referenced to GND and require greater than 4.5 V
on the positive supply for proper operation. If the comparators
are not used, then a split ±2.5 V can be used with the amplifiers
operating normally.
The AD8145 includes a power-down feature that can be
asserted to reduce the supply current when a particular device
is not in use.
BASIC CLOSED-LOOP GAIN CONFIGURATIONS
Each amplifier in the AD8145 comprises two transconductance
amplifiers—one for the input signal and one for negative feedback.
It is important to note that the closed-loop gain of the amplifier
used in the signal path is defined as the single-ended output
voltage of the amplifier divided by its differential input voltage.
Therefore, each amplifier in the AD8145 provides differential-
to-single-ended gain. Additionally, the amplifier used for
feedback has two high impedance inputs—the feedback input,
where the negative feedback is applied, and the REF input,
which can be used as an independent single-ended input to
apply a dc offset to the output signal.
The AD8145 contains on-chip feedback networks between each
amplifier output and its respective feedback input. Closed-loop
gain of an amplifier is set to 1 by connecting the amplifier output
directly to its respective GAIN pin. Doing this places the on-
chip resistors and capacitor in parallel across the amplifier
output and feedback pin. The small feedback capacitor
mitigates the effects of summing-node capacitance, which is
most problematic in the unity gain case. Closed-loop gain of an
amplifier is set to 2 by connecting the respective GAIN pin to a
reference voltage, often directly to ground. In Figure 1, R = 350 Ω
and C = 2 pF.
Some basic gain configurations implemented with an AD8145
amplifier are shown in Figure 33 through Figure 36.
AD8145
+5V
0.01µF
VIN
REF
VREF
R
R
C
OUT
VOUT
GAIN
0.01µF
–5V
Figure 33. Basic Gain = 1 Circuit: VOUT = VIN + VREF
The gain equation for the circuit in Figure 33 is
VOUT = VIN + VREF
(1)
In this configuration, the voltage applied to the REF pin appears
at the output with a gain of 1.
Figure 34 illustrates one way to operate an AD8145 amplifier
with a gain of 2.
+5V
0.01µF
VIN
VREF
REF
C
GAIN
R
R
VOUT
0.01µF
–5V
Figure 34. Basic Gain = 2 Circuit: VOUT = 2(VIN + VREF)
The gain equation for the circuit in Figure 34 is
VOUT = 2(VIN + VREF)
(2)
Rev. 0 | Page 15 of 24
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