LMH6624
,
LMH6626
www.ti.com
SNOSA42G –NOVEMBER 2002–REVISED DECEMBER 2014
7 Detailed Description
7.1 Overview
The LMH6624 and LMH6626 devices are very wide gain bandwidth, ultra low noise voltage feedback operational
amplifiers. Their excellent performances enable applications such as medical diagnostic ultrasound, magnetic
tape & disk storage and fiber-optics to achieve maximum high frequency signal-to-noise ratios. The set of
characteristic plots in Typical Characteristics illustrates many of the performance trade-offs. The following
discussion will demonstrate the proper selection of external components to achieve optimum system
performance.
7.2 Feature Description
7.2.1 Bias Current Cancellation
To cancel the bias current errors of the non-inverting configuration, the parallel combination of the gain setting
(R
g
) and feedback (R
f
) resistors should equal the equivalent source resistance (R
seq
) as defined in Figure 47.
Combining this constraint with the non-inverting gain equation also seen in Figure 47, allows both R
f
and R
g
to
be determined explicitly from the following equations:
R
f
= A
V
R
seq
(1)
R
g
= R
f
/(A
V
-1) (2)
When driven from a 0-Ω source, such as the output of an op amp, the non-inverting input of the LMH6624 and
LMH6626 should be isolated with at least a 25-Ω series resistor.
As seen in Figure 48, bias current cancellation is accomplished for the inverting configuration by placing a
resistor (R
b
) on the non-inverting input equal in value to the resistance seen by the inverting input (R
f
||(R
g
+R
s
)).
R
b
should to be no less than 25 Ω for optimum LMH6624 and LMH6626 performance. A shunt capacitor can
minimize the additional noise of R
b
.
Figure 47. Non-Inverting Amplifier Configuration
Copyright © 2002–2014, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LMH6624 LMH6626
