ADA4891-3 Analog Devices, ADA4891-3 Datasheet - Page 18

• Manufacturer Part Number: ADA4891-3
• Description: Low Cost CMOS, High Speed, Rail-to-Rail Amplifier (Triple)
• Manufacturer: Analog Devices

Specifications of ADA4891-3

• -3db Bandwidth: 220MHz
• Slew Rate: 170V/µs
• Vos: 2.5mV
• Ib: 2pA
• # Opamps Per Pkg: 3
• Input Noise (nv/rthz): 9nV/rtHz
• Vcc-vee: 2.7V to 5.5V
• Isy Per Amplifier: 4.4mA
• Packages: Mini-SO,SOP

ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4
TERMINATING UNUSED AMPLIFIERS
Terminating unused amplifiers in a multiamplifier package is
an important step in ensuring proper operation of the functional
amplifier. Unterminated amplifiers can oscillate and draw
excessive power. The recommended procedure for terminating
unused amplifiers is to connect any unused amplifiers in a
unity-gain configuration and to connect the noninverting input
to midsupply voltage. With symmetrical bipolar power supplies,
this means connecting the noninverting input to ground, as
shown in Figure 59.
In single power supply applications, a synthetic midsupply
source must be created. This can be accomplished with a simple
resistive voltage divider. Figure 60 shows the proper connection
for terminating an unused amplifier in a single-supply
configuration.
DISABLE FEATURE (ADA4891-3 ONLY)
The ADA4891-3 includes a power-down feature that can be
used to save power when an amplifier is not in use. When an
amplifier is powered down, its output goes to a high impedance
state. The output impedance decreases as frequency increases;
this effect can be observed in Figure 34. With the power-down
function, a forward isolation of −40 dB can be achieved at
50 MHz. Figure 46 shows the forward isolation vs. frequency
data. The power-down feature is asserted by pulling the PD1 ,
PD2 , or PD3 pin low.
Table 7 summarizes the operation of the power-down feature.
Table 7. Disable Function
Power-Down Pin Connection (PDx)
>V
<V
TH
TH
Figure 60. Terminating Unused Amplifier with Single Power Supply
or floating
Figure 59. Terminating Unused Amplifier with
Symmetrical Bipolar Power Supplies
2.5kΩ
2.5kΩ
ADA4891
ADA4891
+V
–V
S
S
+V
S
Amplifier Status
Enabled
Disabled
Rev. C | Page 18 of 24
SINGLE-SUPPLY OPERATION
The ADA4891 can also be operated from a single power supply.
Figure 61 shows the ADA4891-3 configured as a single 5 V
supply video driver.
•
•
•
•
The large-signal frequency response obtained with single-
supply operation is identical to the bipolar supply operation
(Figure 18 shows the large-signal frequency response).
Four pairs of low frequency poles are formed by R2/2 and C2,
R3 and C1, R
the −3 dB cutoff frequency at low frequency is 12 Hz. The
values of C1, C2, C5, and C6 can be adjusted to change the low
frequency −3 dB cutoff point to suit individual design needs.
For more information about single-supply operation of op amps,
see the Analog Dialogue article “Avoiding Op Amp Instability
Problems in Single-Supply Applications” (Volume 35, Number 2)
at www.analog.com.
V
IN
50Ω
The input signal is ac-coupled into the amplifier via
Capacitor C1.
Resistor R2 and Resistor R4 establish the input midsupply
reference for the amplifier.
Capacitor C5 prevents constant current from being drawn
through the gain set resistor (R
at dc to provide unity gain to the input midsupply voltage,
thereby establishing the output voltage at midsupply.
Capacitor C6 is the output coupling capacitor.
R1
+5V
22µF
Figure 61. Single-Supply Video Driver Schematic
C1
50kΩ
100kΩ
G
R2
and C5, and R
R3
22µF
453Ω
50kΩ
1µF
C2
R4
C5
R
G
453Ω
R
F
L
+5V
and C6. With this configuration,
–V
0.01µF
10µF
C3
C4
ADA4891-3
S
G
) and enables the ADA4891-3
22µF
C6
R
150Ω
L
V
OUT