ADA4891-3 Analog Devices, ADA4891-3 Datasheet - Page 16

• 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
Table 6. Recommended Component Values and Effect of Gain on ADA4891-3/ADA4891-4 Performance (R
Gain
−1
+1
+2
+5
+10
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 resistors.
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 52 and Figure 53 show the effect of using various values
for Feedback Resistor R
Figure 52 shows the effect for the ADA4891-1/ADA4891-2.
Figure 53 show the effect for the ADA4891-3/ADA4891-4.
Note that a larger R
additional pole formed by R
shifts down in frequency and interacts significantly with the
internal poles of the amplifier.
Figure 52. 0.1 dB Gain Flatness, Noninverting Gain Configuration,
–0.2
–0.3
–0.4
–0.1
0.2
0.1
0
0.1
V
G = +2
V
R
R
453
0
453
453
453
S
OUT
L
F
= 5V
= 150Ω
(Ω)
Feedback Network Values
= 2V p-p
F
ON 0.1 dB GAIN FLATNESS
F
value causes more peaking because the
ADA4891-1/ADA4891-2
F
on the 0.1 dB gain flatness of the parts.
1
FREQUENCY (MHz)
R
R
F
G
G
and the input stray capacitance
R
453
Open
453
90.6
45.3
= R
= R
R
G
G
(Ω)
F
F
= R
= 698Ω
= 604Ω
F
= 549Ω
10
R
G
= R
F
−3 dB Small-Signal Bandwidth (MHz)
V
97
220
97
31
13
= 649Ω
OUT
= 200 mV p-p
100
Rev. C | Page 16 of 24
To obtain the desired 0.1 dB bandwidth, adjust the feedback
resistor, R
be adjusted, a small capacitor can be placed in parallel with R
to reduce peaking.
The feedback capacitor, C
resistor, which cancels out the pole formed by the input stray
capacitance and the gain and feedback resistors. For a first pass
in determining the C
where:
R
C
R
C
Using this equation, 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 54 shows the effect of using various values for the
feedback capacitor to reduce peaking. In this case, the
ADA4891-1/ ADA4891-2 are used for demonstration purposes
and R
the board parasitics, is approximately 2 pF.
G
S
F
F
is the input stray capacitance.
is the feedback resistor.
is the feedback capacitor.
is the gain resistor.
R
Figure 53. 0.1 dB Gain Flatness, Noninverting Gain Configuration,
F
G
–0.2
–0.3
–0.4
–0.5
–0.1
0.3
0.2
= R
0.1
× C
0
0.1
F
, as shown in Figure 52 and Figure 53. If R
G
V
G = +2
V
R
S
S
OUT
L
= 604 Ω. The input stray capacitance, together with
= R
= 5V
= 150Ω
= 2V p-p
F
× C
t
186
151
181
112
68
R
F
F
ADA4891-3/ADA4891-4
value, use the following equation:
Slew Rate (V/μs)
1
F
FREQUENCY (MHz)
, forms a zero with the feedback
R
G
= R
R
F
G
t
194
262
223
120
67
= 357Ω
= R
R
F
F
G
F
is determined empirically.
= R
= 402Ω
L
F
= 1 kΩ)
= 301Ω
10
R
G
= R
Peaking (dB)
0.9
4.1
0.9
0
0
F
= 453Ω
F
cannot
100
F