ADA4891-2ARZ Analog Devices Inc, ADA4891-2ARZ Datasheet - Page 13

• Manufacturer Part Number: ADA4891-2ARZ
• Description: Lw Cst CMOS Hgh Spd RR Amp
• Manufacturer: Analog Devices Inc

Specifications of ADA4891-2ARZ

• Amplifier Type: General Purpose
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 210 V/µs
• -3db Bandwidth: 240MHz
• Current - Input Bias: 2pA
• Voltage - Input Offset: 2500µV
• Current - Supply: 4.4mA
• Current - Output / Channel: 150mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V, ±1.35 V ~ 2.75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Gain Bandwidth Product: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

EFFECT OF R
Gain flatness is an important specification in video applications.
It represents the maximum allowable deviation in the signal
amplitude within the pass band. Tests have revealed that the
human eye is unable to distinguish brightness variations of
less than 1%, which translates into a 0.1 dB signal drop within
the pass band, or put simply, 0.1 dB gain flatness.
The PCB layout configuration and bond pads of the chip often
contribute to stray capacitance. The stray capacitance at the
inverting input forms a pole with the feedback and gain resistor.
This additional pole adds phase shift and reduces phase margin
in the closed-loop phase response, causing instability in the
amplifier and peaking in the frequency response.
Figure 43 shows the effect of using various values of Feedback
Resistor R
causes more peaking because the additional pole formed by R
and the input stray capacitance, shifts down in frequency and
interacts significantly with the internal poles of the amplifier.
To get the desired 0.1 dB bandwidth, adjust the feedback
resistor, R
small capacitor can be placed in parallel with R
peaking.
–0.2
–0.3
–0.4
–0.1
0.2
0.1
0
0.1
F
F
, as shown in Figure 43. If R
on the 0.1 dB gain flatness. Note that a larger R
V
G = +2
V
R
S
OUT
L
= +5V
= 150Ω
= 2V p-p
F
Figure 43. Noninverting Configuration
ON 0.1 dB GAIN FLATNESS
1
FREQUENCY (MHz)
R
R
G
G
= R
= R
R
G
F
F
= R
= 604Ω
= 698Ω
F
= 549Ω
F
cannot be adjusted, a
10
R
G
= R
F
to reduce
F
= 649Ω
F
100
value
Rev. A | Page 13 of 20
F
,
The feedback capacitor, C
which cancels out the pole formed by the input stray capacitance
and the gain and feedback resistor. For a first pass in determining
the C
gain resistor, C
resistor, and C
where the original closed-loop frequency response of the amplifier
is restored as if there is no stray input capacitance. Most often,
however, the value of C
Figure 44 shows the effect of using various values for the feedback
capacitors to reduce peaking. In this case, R
input stray capacitance, together with the board parasitics, is
approximately 2 pF.
F
value, use the equation R
0.2
0.1
0.1
0.2
0.3
0
0.1
V
G = 2
R
R
V
S
OUT
F
L
= 5V
= 604Ω
= 150Ω
Figure 44. 0.1 dB Gain Flatness vs. C
F
S
= 2V p-p
is the feedback capacitor. This is the condition
is the input stray capacitance, R
F
is determined empirically.
F
1
, forms a zero with the feedback resistor,
FREQUENCY (MHz)
ADA4891-1/ADA4891-2
G
C
× C
F
= 3.3pF
S
= R
10
F
C
F
× C
F
, V
F
= R
= 0pF
S
= 5 V
F
, where R
F
G
is the feedback
C
= 604 Ω. The
F
= 1pF
100
G
is the