lmh6629sdx National Semiconductor Corporation, lmh6629sdx Datasheet - Page 13

• Manufacturer Part Number: lmh6629sdx
• Description: Lmh6629 Ultra-low Noise, High Speed Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Application Section
INTRODUCTION
The LMH6629 is a very wide gain bandwidth, ultra low-noise
voltage feedback operational amplifier. The excellent noise
and bandwidth enables applications such as medical diag-
nostic ultrasound, magnetic tape & disk storage and fiberop-
tics to achieve maximum high frequency signal-to-noise
ratios. The following discussion will enable the proper selec-
tion of external components to achieve optimum system per-
formance.
The LMH6629 has some additional features to allow maxi-
mum performance. As shown in
for low power shut down and two internal compensation set-
tings, which are further discussed below. Also provided is a
feedback (FB) pin which allows the placement of the feedback
resistor directly adjacent to the inverting input (IN-) pin. This
pin simplifies board layout and minimizes the possibility of
unwanted interaction between the feedback path and other
circuit elements.
The LLP-8 package requires the bottom-side Die Attach Pad-
dle (DAP) to be soldered to the circuit board for proper thermal
dissipation and to get the thermal resistance number speci-
fied. The DAP is tied to the V
package. Thus, the circuit board copper area devoted to DAP
heatsinking connection should be at the V- potential as well.
Please refer to the package drawing for the recommended
land pattern and recommended DAP connection dimensions.
FIGURE 3. 8-Pin LLP SDA088AD (Top View)
FIGURE 2. 8-Pin LLP Pinout Diagram
-
potential within the LMH6629
Figure 2
there are provisions
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CONTROL PINS
The LMH6629 has two digital control pins; PD and COMP
pins. The PD pin, used for powerdown, floats high (on) when
not driven. When the PD pin is pulled low, the amplifier is dis-
abled and the amplifier output stage goes into a high
impedance state so the feedback and gain set resistors de-
termine the output impedance of the circuit. The other control
pin, the COMP pin, allows control of the internal compensa-
tion and defaults to the lower gain mode or logic 0.
COMPENSATION
Nearly all high-speed operational amplifiers are now internally
compensated. To use external compensation capacitors
would compromise stability and performance due to bond
wire and board parasitic reactances. The LMH6629 gives a
degree of flexibility that was lost with on chip compensation.
There are two compensation settings that can be controlled
by the COMP pin. The default setting is set through an internal
pull down resistor and places the COMP pin at the logic 0
state. In this configuration the on chip compensation is set to
the maximum and bandwidth is reduced to enable stability at
gains as low as 4V/V.
When this pin is driven to the logic 1 state the internal com-
pensation is decreased to allow higher bandwidth at higher
gains. In this state the minimum stable gain is 10V/V. Due to
the reduced compensation slew rate and large signal band-
width are significantly enhanced for the higher gains.
BIAS CURRENT CANCELLATION
The LMH6629 offers exceptional offset voltage accuracy. In
order to preserve the low offset voltage errors, care must be
taken to avoid voltage errors due to input bias currents. This
is important in both inverting and non inverting applications.
The non-inverting circuit is used here as an example. To can-
cel the bias current errors of the non-inverting configuration,
the parallel combination of the gain setting (R
(R
(R
the non-inverting gain equation also seen in
both R
equations:
f
seq
) resistors should equal the equivalent source resistance
) as defined in
f
and R
g
R
to be determined explicitly from the following
f
= A
Figure
V
R
seq
and R
4. Combining this constraint with
g
= R
f
/(A
V
-1)
g
Figure 4
) and feedback
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allows