Datasheet

LT1210
9
1210fb
For more information www.linear.com/LT1210
applicaTions inForMaTion
The LT1210 is a current feedback amplifier with high output
current drive capability. The device is stable with large
capacitive loads and can easily supply the high currents
required by capacitive loads. The amplifier will drive low
impedance loads such as cables with excellent linearity
at high frequencies.
Feedback Resistor Selection
The optimum value for the feedback resistors is a function
of the operating conditions of the device, the load imped
-
ance and the desired flatness of response. The Typical AC
Performance tables give the values which result in less than
1dB of peaking for various resistive loads and operating
conditions. If this level of flatness is not required, a higher
bandwidth can be obtained by use of a lower feedback
resistor. The characteristic curves of Bandwidth vs Supply
Voltage indicate feedback resistors for peaking up to 5dB.
These curves use a solid line when the response has less
than 1dB of peaking and a dashed line when the response
has 1dB to 5dB of peaking. The curves stop where the
response has more than 5dB of peaking.
For resistive loads, the COMP pin should be left open (see
Capacitive Loads section).
Capacitive Loads
The LT1210 includes an optional compensation network
for driving capacitive loads. This network eliminates most
of the output stage peaking associated with capacitive
loads, allowing the frequency response to be flattened.
Figure 1 shows the effect of the network on a 200pF load.
Without the optional compensation, there is a 6dB peak
at 40MHz caused by the effect of the capacitance on the
output stage. Adding a 0.01µF bypass capacitor between
the output and the COMP pins connects the compensation
and greatly reduces the peaking. A lower value feedback
resistor can now be used, resulting in a response which
is flat to ±1dB to 40MHz. The network has the greatest
effect for C
L
in the range of 0pF to 1000pF. The graphs of
Bandwidth and Feedback Resistance vs Capacitive Load
can be used to select the appropriate value of feedback
resistor. The values shown are for 1dB and 5dB peaking
at a gain of 2 with no resistive load. This is a worst-case
condition, as the amplifier is more stable at higher gains
and with some resistive load in parallel with the capacitance.
Also shown is the –3dB bandwidth with the suggested
feedback resistor vs the load capacitance.
Although the optional compensation works well with ca
-
pacitive loads, it simply reduces the bandwidth when it is
connected with resistive loads.
For instance, with a 10Ω
load, the bandwidth drops from 35MHz to 26MHz when the
compensation is connected. Hence, the compensation was
made optional. To disconnect the optional compensation,
leave the COMP pin open.
Shutdown/Current Set
If the shutdown feature is not used, the SHUTDOWN pin
must be connected to ground or V
–
.
The Shutdown pin can be used to either turn off the bias-
ing for the amplifier, reducing the quiescent current to
less than 200µA, or to control the quiescent current in
normal operation.
The total bias current in the L
T1210 is controlled by the
current flowing out of the Shutdown pin. When the Shut
-
down pin is
open or driven to the positive supply, the part
is shut down. In the shutdown mode, the output looks
like a 70pF capacitor and the supply current is typically
less than 100µA. The Shutdown pin is referenced to the
positive
supply through an
internal bias circuit (see the
Simplified Schematic). An easy way to force shutdown is
to use open-drain (collector) logic. The circuit shown in
Figure 2 uses a 74C904 buffer to interface between 5V
logic and the LT1210. The switching time between the
active and shutdown states is about 1µs.
A 24k pull-up
Figure 1
FREQUENCY (MHz)
1
–6
VOLTAGE GAIN (dB)
–2
2
6
10
10 100
1210 F01
–4
0
4
8
12
14
V
S
= ±15V
C
L
= 200pF
R
F
= 1.5k
COMPENSATION
R
F
= 3.4k
NO COMPENSATION
R
F
= 3.4k
COMPENSATION
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.