LMH6504 Datasheet, PDF (15/29 Pages) National Semiconductor (TI) – Wideband, Low Power, Variable Gain Amplifier

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LMH6504
SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013
THEORETICAL GAIN
MAX GAIN LIMIT
MIN GAIN LIMIT
D
TYPICAL GAIN
C
B
PARAMETER:
GAIN ACCURACY (TYPICAL) = B-C
A
GAIN ACCURACY (MAX/MIN) = (D-C)/(A-C)
GAIN MATCHING (MAX/MIN) = (D-B)/(A-B)
V_G (V)
Figure 43. LMH6504 Gain Accuracy & Gain Matching Defined
GAIN PARTITIONING
If high levels of gain are needed, gain partitioning should be considered:
V_G
V_IN
+
LMH6624
-
25:
1
2
R_C
6
LMH6504
V_O
3
7
4
R₂
R_F
R_G
R₁
Figure 44. Gain Partitioning
The maximum gain range for this circuit is given by the following equation:
R₂ R_F
MAXIMUM GAIN = 1 + ·
·K
R₁
R_G
(4)
The LMH6624 is a low noise wideband voltage feedback amplifier. Setting R₂ at 909Ω and R₁ at 100Ω produces
a gain of 20 dB. Setting R_F at 1000Ω as recommended and R_G at 50Ω, produces a gain of about 26 dB in the
LMH6504. The total gain of this circuit is therefore approximately 46 dB. It is important to understand that when
partitioning to obtain high levels of gain, very small signal levels will drive the amplifiers to full scale output. For
example, with 46 dB of gain, a 20 mV signal at the input will drive the output of the LMH6624 to 200 mV, the
output of the LMH6504 to 4V. Accordingly, the designer must carefully consider the contributions of each stage
to the overall characteristics. Through gain partitioning the designer is provided with an opportunity to optimize
the frequency response, noise, distortion, settling time, and loading effects of each amplifier to achieve improved
overall performance.
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