LMH6618_0711 Datasheet, PDF (24/28 Pages) National Semiconductor (TI) – PowerWise® 130 MHz, 1.25 mA RRIO Operational Amplifiers

CURRENT SENSE AMPLIFIER
With it’s rail-to-rail input and output capability, low V_OS, and
low I_B the LMH6618 is an ideal choice for a current sense
(1)
amplifier application. Figure 9 shows the schematic of the
LMH6618 set up in a low-side sense configuration which pro-
vides a conversion gain of 2V/A. Voltage error due to V_OS can
(2)
be calculated to be V_OS x (1 + R_F/R_G) or
0.6 mV x 21 = 12.6 mV. Voltage error due to I_O is I_O x R_F or
0.26 µA x 1 kΩ = 0.26 mV. Hence total voltage error is
12.6 mV + 0.26 mV or 12.86 mV which translates into a cur-
rent error of 12.86 mV/(2 V/A) = 6.43 mA.
20195841
FIGURE 9. Current Sense Amplifier
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.
20195862
FIGURE 10. Photodiode Modeled with Capacitance
Elements
Figure 10 shows the LMH6618 modeled with photodiode and
the internal op amp capacitances. The LMH6618 allows cir-
cuit operation of a low intensity light due to its low input bias
current by using larger values of gain (R_F). The total capaci-
tance (C_T) on the inverting terminal of the op amp includes
the photodiode capacitance (C_PD) and the input capacitance
of the op amp (C_IN). This total capacitance (C_T) plays an im-
portant role in the stability of the circuit. The noise gain of this
circuit determines the stability and is defined by:
20195865
FIGURE 11. Bode Plot of Noise Gain Intersecting with Op
Amp Open-Loop Gain
Figure 11 shows the bode plot of the noise gain intersecting
the op amp open loop gain. With larger values of gain, C_T and
R_F create a zero in the transfer function. At higher frequencies
the circuit can become unstable due to excess phase shift
around the loop.
A pole at f_P in the noise gain function is created by placing a
feedback capacitor (C_F) across R_F. The noise gain slope is
flattened by choosing an appropriate value of C_F for optimum
performance.
Theoretical expressions for calculating the optimum value of
C_F and the expected −3 dB bandwidth are:
(3)
(4)
Equation 4 indicates that the −3 dB bandwidth of the TIA is
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 3 shows the measurement results of the LMH6618 with
different photodiodes having various capacitances (C_PD) and
a feedback resistance (R_F) of 1 kΩ.
www.national.com
24