551600075-001 National Semiconductor, 551600075-001 Datasheet - Page 27

• Manufacturer Part Number: 551600075-001
• Description: BOARD FOR SOIC LMH6612/19
• Manufacturer: National Semiconductor
• Series: WEBENCH® Buildit Boardr

Specifications of 551600075-001

• Channels Per Ic: 2 - Dual
• Amplifier Type: General Purpose
• Board Type: Bare (Unpopulated)
• Utilized Ic / Part: LMH6612, LMH6619
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Output Type: -
• Current - Output / Channel: -
• Voltage - Supply, Single/dual (±): -
• -3db Bandwidth: -
• Slew Rate: -
• Current - Supply (main Ic): -

CURRENT SENSE AMPLIFIER AND OPTIMIZING
ACCURACY IN PRECESION APPLICATIONS
With it’s rail-to-rail output capability, low V
LMH6611 is an ideal choice for a current sense amplifier ap-
plication.
up in a low-side sense configuration which provides a con-
version gain of 2V/A. Voltage error due to V
lated to be V
Voltage error due to I
Hence worst case total voltage error is 12.6 mV + 0.5 mV or
13.1 mV which translates into a current error of 13.1 mV/(2 V/
A) = 6.55 mA.
This circuit employs DC source resistance matching at the
two input terminals in order to minimize the output DC error
caused by input bias current. Another technique to reduce
output offset in a non-inverting amplifier configuration is to in-
troduce a DC offset current into the inverting input of the
amplifier. To ensure minimal impact on frequency response
be sure to inject the DC offset current through large resistors.
Conversely if optimizing an inverting amplifier configuration
simply apply offset adjustment to the non-inverting input.
TRANSIMPEDANCE AMPLIFIER
By definition, a photodiode produces either a current or volt-
age output from exposure to a light source. A Tran-
simpedance Amplifier (TIA) is utilized to convert this low-level
current to a usable voltage signal. The TIA often will need to
be compensated to insure proper operation.
Figure 12
the internal op amp capacitances. The LMH6611 allows cir-
cuit operation of a low intensity light due to its low input bias
FIGURE 12. Photodiode Modeled with Capacitance
Figure 11
shows the LMH6611 modeled with photodiode and
FIGURE 11. Current Sense Amplifier
OS
x (1 + R
shows the schematic of the LMH6611 set
O
is I
Elements
F
O
/R
x R
G
) or 0.6 mV x 21 = 12.6 mV.
F
or 0.5 µA x 1 kΩ = 0.5 mV.
OS
OS
, and low I
can be calcu-
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B
the
27
current by using larger values of gain (R
tance (C
the photodiode capacitance (C
of the op amp (C
portant role in the stability of the circuit. The noise gain of this
circuit determines the stability and is defined by:
FIGURE 13. Bode Plot of Noise Gain Intersecting with Op
Figure 13
the op amp open loop gain. With larger values of gain, C
R
the circuit can become unstable due to excess phase shift
around the loop.
A pole at f
feedback capacitor (C
flattened by choosing an appropriate value of C
performance.
Theoretical expressions for calculating the optimum value of
C
Equation 4
inversely proportional to the feedback resistor. Therefore, if
the bandwidth is important then the best approach would be
to have a moderate transimpedance gain stage followed by a
broadband voltage gain stage.
Table 4
different photodiodes having various capacitances (C
a feedback resistance (R
F
F
create a zero in the transfer function. At higher frequencies
and the expected −3 dB bandwidth are:
shows the measurement results of the LMH6611 with
T
) on the inverting terminal of the op amp includes
shows the bode plot of the noise gain intersecting
P
indicates that the −3 dB bandwidth of the TIA is
in the noise gain function is created by placing a
IN
Amp Open Loop Gain
). This total capacitance (C
F
) across R
F
) of 1 kΩ.
PD
) and the input capacitance
F
. The noise gain slope is
F
). The total capaci-
T
) plays an im-
F
www.national.com
for optimum
PD
30033665
T
) and
and
(1)
(2)
(3)
(4)