![](http://datasheet.mmic.net.cn/Intersil/HA1-2556-9_datasheet_104713/HA1-2556-9_8.png)
8
FN2477.7
April 25, 2013
Each input X, Y and Z have similar wide bandwidth and input
characteristics. This is unlike earlier products where one
input was dedicated to a slow moving control function as is
required for Automatic Gain Control. The HA-2556 is
versatile enough for both.
Although the X and Y inputs have similar AC characteristics,
they are not the same. The designer should consider input
parameters such as small signal bandwidth, AC feedthrough
and 0.1dB gain flatness to get the most performance from
the HA-2556. The Y-Channel is the faster of the two inputs
with a small signal bandwidth of typically 57MHz vs 52MHz
for the X-Channel. Therefore in AM Signal Generation, the
best performance will be obtained with the Carrier applied to
the Y-Channel and the modulation signal (lower frequency)
applied to the X-Channel.
Scale Factor Control
The HA-2556 is able to operate over a wide supply voltage
range ±5V to ±17.5V. The ±5V range is particularly useful in
video applications. At ±5V the input voltage range is reduced
to ±1.4V. The output cannot reach its full scale value with
this restricted input, so it may become necessary to modify
the scale factor. Adjusting the scale factor may also be
useful when the input signal itself is restricted to a small
portion of the full scale level. Here, we can make use of the
high gain output amplifier by adding external gain resistors.
Generating the maximum output possible for a given input
signal will improve the Signal-to-Noise Ratio and Dynamic
Range of the system. For example, let’s assume that the
input signals are 1VPEAK each then, the maximum output for
the HA-2556 will be 200mV. (1V x 1V)/(5V) = 200mV. It
would be nice to have the output at the same full scale as
our input, so let’s add a gain of 5 as shown in Figure
9.One caveat is that the output bandwidth will also drop by this
factor of 5. The multiplier equation then becomes
Current Output
Another useful circuit for low voltage applications allows the
user to convert the voltage output of the HA2556 to an output
current. The HA-2557 is a current output version offering
100MHz of bandwidth, but its scale factor is fixed and does not
have an output amplifier for additional scaling. Fortunately, the
circuit in Figure
10 provides an output current that can be
scaled with the value of RCONVERT and provides an output
impedance of typically 1M
Ω. IOUT then becomes Equation 16: Video Fader
The Video Fader circuit provides a unique function. Here
Channel B is applied to the minus Z input in addition to the
minus Y input. In this way, the function in Figure
11 is
generated. VMIX will control the percentage of Channel A and
Channel B that are mixed together to produce a resulting
video image or other signal.
The Balance equation looks like Equation
17:Which simplifies to Equation
18:When VMIX is 0V the equation becomes VOUT = ChB and
ChA is removed, conversely when VMIX is 5V the equation
becomes VOUT = ChA eliminating ChB. For VMIX values 0V
≤ V
MIX ≤ 5V the output is a blend of ChA and ChB.
Other Applications
As previously shown, a function may contain several
different operators at the same time and use only one
HA-2556
1/5V
X
Y
VOUT
Z
VX+
VX-
VY+
VY-
VZ+
VZ-
∑
W
A
B
1k
Ω
250
Ω
RF
RG
ExternalGain
R
F
R
G
--------
1
+
=
+
-
+
-
A
+
-
+
-
FIGURE 9. EXTERNAL GAIN OF 5
W
5AB
5
------------
AB
×
==
(EQ. 15)
I
OUT
AB
×
5
--------------
1
R
CONVERT
--------------------------------
×
=
(EQ. 16)
HA-2556
1/5V
X
Y
VOUT
Z
VX+
VX-
VY+
VY-
VZ+
VZ-
∑
IOUT
A
B
RCONVERT
+
-
+
-
A
+
-
+
-
FIGURE 10. CURRENT OUTPUT
V
MIX
()
ChA
ChB
–
()
×
5V
OUT
ChB
–
()
=
(EQ. 17)
V
OUT
ChB
V
MIX
5
-------------- ChA
ChB
–
()
+
=
(EQ. 18)
NC
VY+
-15V
VOUT
+15V
VX +
NC
50
Ω
NC
VZ-
VZ+
ChA
ChB
VY-
VMIX
(0V TO 5V)
14
15
16
9
13
12
11
10
1
2
3
4
5
7
6
8
Σ
+ -
REF
+
-
+
-
+
-
FIGURE 11. VIDEO FADER
HA-2556