LMH6554_15 Datasheet, PDF (16/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
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In order to ensure stability resistance should be added between the capacitive load and the amplifier output pins.
The value of the resistor is dependent on the amount of capacitive load as shown in Figure 29. This resistive
value is a suggestion. System testing will be required to determine the optimal value. Using a smaller resistor will
retain more system bandwidth at the expense of overshoot and ringing, while larger values of resistance will
reduce overshoot but will also reduce system bandwidth.
3
2
1
CL=2.2 pF, RO=38:
0
-1
-2 CL=6.8 pF, RO=22:
-3
-4 C_L=18 pF, R_O=14:
-5
-6 C_L=68 pF, R_O=5:
-7
-8
-9 _{VOD = 200 mVPP}
-10
1
10
100
1000 10000
FREQUENCY (MHz)
Figure 28. Frequency Response vs Capacitive Load
70
LOAD = 1k: || CAP LOAD
60
50
40
30
20
10
0
5 10 15 20 25 30 35 40
CAPACITIVE LOAD (pF)
Figure 29. Suggested R_OUT vs Capacitive Load
9.2.2 Fully Differential Operation
The LMH6554 will perform best in a fully differential configuration. The circuit shown in Figure 30 is a typical fully
differential application circuit as might be used to drive an analog to digital converter (ADC). In this circuit the
closed loop gain is A_V= V_OUT / V_IN = R_F / R_G, where the feedback is symmetric. The series output resistors, R_O,
are optional and help keep the amplifier stable when presented with a capacitive load. Refer to the Driving
Capacitive Loads section for details.
Here is the expression for the input impedance, R_IN, as defined in Figure 30:
R_IN = 2R_G
When driven from a differential source, the LMH6554 provides low distortion, excellent balance, and common
mode rejection. This is true provided the resistors R_F, R_G and R_O are well matched and strict symmetry is
observed in board layout. With an intrinsic device CMRR of greater than 70 dB, using 0.1% resistors will give a
worst case CMRR of around 50 dB for most circuits.
The circuit configuration shown in Figure 30 was used to measure differential S-parameters in a 100Ω
environment at a gain of 1 V/V. Refer to Figure 24 in Typical Performance Characteristics V_S = ±2.5 V for
measurement results.
200:
R_S
67:
50:
V_S
a
+
V_IN
R_IN
-
R_S
50: ^67:
200:
R_{G VCM}
R_G
200:
R_F
+
LMH6554
-
V_EN
R_F
50:
-
VOUT
+
R_L=100:
50:
200:
Figure 30. Differential S-Parameter Test Circuit
16
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