LMH6504 Datasheet, PDF (14/29 Pages) National Semiconductor (TI) – Wideband, Low Power, Variable Gain Amplifier

LMH6504
SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013
www.ti.com
As the I_{RG_MAX} limit is approached (with increasing input voltage or with lowering of R_G), the device harmonic
distortion will increase. Changes in R_F will have a dramatic effect on the small signal bandwidth. The output
amplifier of the LMH6504 is a current feedback amplifier (CFA) and its bandwidth is determined by R_F. As with
any CFA, doubling the feedback resistor will roughly cut the bandwidth of the device in half. For more about
CFA’s, see the basic tutorial, OA-20, “Current Feedback Myths Debunked” (SNOA376), or a more rigorous
analysis, OA-13, “Current Feedback Amplifier Loop Gain Analysis and Performance Enhancements” (SNOA366).
OTHER CONFIGURATIONS
1) Single Supply Operation
The LMH6504 can be configured for use in a single supply environment. Doing so requires the following:
a. Bias pin 4 and R_G to a “virtual half supply” somewhere close to the middle of V⁺ and V^-range. The other end
of R_G is tied to pin 3. The “virtual half supply” needs to be capable of sinking and sourcing the expected
current flow through R_G.
b. Ensure that V_G can be adjusted from 0V to 2V above the “virtual half supply”.
c. Bias the input (pin 2) to make sure that it stays within the range of 1.8V above V^-to 1.8V below V⁺ (see “Input
voltage Range” specification in the Electrical Characteristics table). This can be accomplished by either DC
biasing the input and AC coupling the input signal, or alternatively, by direct coupling if the output of the
driving stage is also biased to half supply.
Arranged this way, the LMH6504 will respond to the current flowing through R_G. The gain control relationship will
be similar to the split supply arrangement with V_G measured referenced to pin 4. Keep in mind that the circuit
described above will also center the output voltage to the “virtual half supply voltage”.
2) Arbitrarily Referenced Input Signal
Having a wide input voltage range on the input (pin 2) (+/-3.2V typical), the LMH6504 can be configured to
control the gain on signals which are not referenced to ground (e.g. Half Supply biased circuits, etc.). We will call
this node the “reference node”. In such cases, the other end of R_G (the side not tied to pin 3) can be tied to this
reference node so that R_G will “look at” the difference between the signal and this reference only. Keep in mind
that the reference node needs to source and sink the current flowing through R_G.
Application Information
GAIN ACCURACY
Gain accuracy is defined as the actual gain compared against the theoretical gain at a certain V_G (results
expressed in dB) (See Figure 43).
Theoretical gain is given by:
R_F
A(V/V) = K
xx
R_G
1
N - V_G
1 + e ^VC
(3)
Where K = 0.965 (nominal) N = 0.96V & V_C = 80mV @ room temperature
For a V_G range, the value specified in the tables represents the worst case accuracy over the entire range. The
"Typical" value would be the worst case difference between the "Typical gain" and the "Theoretical gain". The
"Max" value would be the worst case difference between the actual gain and the "Theoretical gain" for the entire
population.
GAIN MATCHING
Gain matching as the limit on gain variation at a certain V_G (expressed in dB) (see Figure 43) and is specified as
"Max" only (no "Typical"). For a V_G range, the value specified represents the worst case matching over the entire
range. The "Max" value would be the worst case difference between the actual gain and the typical gain for the
entire population.
14
Submit Documentation Feedback
Product Folder Links: LMH6504
Copyright © 2003–2013, Texas Instruments Incorporated