LTC6268-10_15 Datasheet, PDF (14/22 Pages) Linear Technology – 4GHz Ultra-Low Bias Current FET Input Op Amp

LTC6268-10/LTC6269-10
APPLICATIONS INFORMATION
Achieving Higher Bandwidth with Higher Gain TIAs
Good layout practices are essential to achieving best
results from a TIA circuit. The following two examples
show drastically different results from an LTC6268-10 in
a 402k TIA. (See Figure 4.) The first example is with an
0805 resistor in a basic circuit layout. In a simple layout,
without expending a lot of effort to reduce feedback ca-
pacitance, the rise time achieved is about 87ns (Figure 5),
implying a bandwidth of 4MHz (BW = 0.35/t_r). In this case,
the bandwidth of the TIA is limited not by the GBW of
the LTC6268-10, but rather by the fact that the feedback
capacitance is reducing the actual feedback impedance
(the TIA gain itself) of the TIA. Basically, it’s a resistor
bandwidth limitation. The impedance of the 402kΩ is being
reduced by its own parasitic capacitance at high frequency.
From the 4MHz bandwidth and the 402k low frequency
gain, we can estimate the total feedback capacitance as
C = 1/(2π • 4MHz • 402kΩ) = 0.1pF. That’s fairly low, but
it can be reduced further.
With some extra layout techniques to reduce feedback
capacitance, the bandwidth can be increased. Note that
we are increasing the effective “bandwidth” of the 402k
resistance. A very powerful method to reduce feedback
capacitance is to shield the E field paths that give rise to
the capacitance. In this particular case, the method is to
place a ground trace between the resistor pads. Such a
ground trace shields the output field from getting to the
summing node end of the resistor and effectively shunts
the field to ground instead. The trace increases the output
load capacitance very slightly. See Figure 6 for a pictorial
representation.
Figure 7 shows the dramatic increase in bandwidth simply
by careful attention to low capacitance methods around
the feedback resistance. Bandwidth and rise time went
from 4MHz (87ns) to 34MHz (10.3ns), a factor of 8. The
ground trace used for LTC6268-10 was much wider than
that used in the case of the LTC6268 (see LTC6268 data
sheet), extending under the entire resistor dielectric. As-
suming all the bandwidth limit is due to feedback capaci-
tance (which isn't fair), we can calculate an upper limit of
C_f = 1/(2π • 402kΩ • 34MHz) = 11.6fF.
PARASITIC
FEEDBACK C
402k
I_PD
CASE
K
PD
A
–2.5
+2.5
–
LTC6268-10
+
–2.5
V_OUT
626810 F04
PD: OSI FCI-125G-006
Figure 4. LTC6268-10 and Low Capacitance Photodiode
in a 402kΩ TIA
LASER DRIVE
(2mA/DIV)
OUTPUT
(500MV/DIV)
20ns/DIV
626810 F05
Figure 5. Time Domain Response of 402kΩ TIA without
Extra Effort to Reduce Feedback Capacitance. Rise Time
Is 87ns and BW Is 4MHz
14
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626810f