ltc6244hv Linear Technology Corporation, ltc6244hv Datasheet - Page 17

• Manufacturer Part Number: ltc6244hv
• Description: Dual 50mhz, Low Noise, Rail-to-rail, Cmos Op Amp
• Manufacturer: Linear Technology Corporation

The solution is as shown in the circuit of Figure 6a, which
uses a capacitor-resistor pair to enable the AC benefi ts of
bootstrapping while allowing a different reverse DC voltage
on the photodiode. The JFET is still running at the same
current, but an arbitrary reverse bias may be applied to
the photodiode. The output noise spectrum of the circuit
with 0V of photodiode reverse bias is shown in Figure 6b.
Photodiode capacitance is again 3650pF, as in the original
circuit of Figure 4a. This noise plot with 0V bias shows
that bootstrapping alone was responsible for a factor of
6.2 noise reduction, from 1800nV/√Hz to 291nV/√Hz at
10kHz, independent of photodiode capacitance. However,
photodiode capacitance can now can be reduced arbitrarily
APPLICATIO S I FOR ATIO
–5V
Figure 5b: Output Noise Spectral Density of Figure 5a. The
Simple JFET Bootstrap Improves Noise (and Bandwidth)
Drastically. Noise Density at 10kHz is Now 220nV/√Hz, About
a 8.2x Reduction. This is Mostly Due to the Bootstrap Effect
of Swapping the 1nV/√Hz of the JFET for the 7.8nV/√Hz of the
Op Amp
R
4.99k
BIAS
S1227-1010BQ
HAMAMATSU
PHOTODIODE
C
LARGE AREA
5V
Figure 5a. Large Area Diode Bootstrapping
PD
1k
= 3000pF
PHILIPS
BF862
JFET
I
PD
U
FREQUENCY (Hz)
–
LTC6244HV
+
U
1/2
–5V
10k
5V
0.25pF
6244 F04a
1M
C
R
F
F
W
V
BW = 350kHz
OUTPUT NOISE = 220nV/√Hz
OUT
V
OUT
= 1M • I
6244 F05b
100k
PD
U
AT10kHz
by providing reverse bias, and the photodiode can also be
reversed to support either cathode or anode connections
for positive or negative going outputs.
The circuit on the last page of this data sheet shows fur-
ther reduction in noise by paralleling four JFETs to attain
152nV/√Hz at 10kHz, a noise of 12 times less than the
basic photodiode circuit of Figure 4a.
–5V
V
Figure 6a. The Addition of a Capacitor and Resistor Enable the
Benefi t of Bootstrapping While Applying Arbitrary Photodiode
Bias Voltage V
BB
Figure 6b: Output Spectrum of Circuit of Figure 6a, with
Photodiode Bias at 0V. Photodiode Capacitance is Back Up,
as in the Original Circuit of Figure 4a. However, it can be
Reduced Arbitrarily by Providing Reverse Bias. This Plot
Shows that Bootstrapping Alone Reduced the 10kHz Noise
Density by a Factor of 6.2, from 1800nV/√Hz to 291nV/√Hz.
4.7µF
4.99k
4.99k
X7R
HAMAMATSU LARGE AREA
5V
S1227-1010BQ
PHOTODIODE
C
1k
PD
= 3000pF
PHILIPS
I
BF862
BB
PD
JFET
FREQUENCY (Hz)
–
LTC6244HV
+
1/2
10k
–5V
5V
0.25pF
6244 F06a
1M
C
R
F
F
V
BW = 250kHz
OUTPUT NOISE = 291nV/√Hz
OUT
LTC6244
V
OUT
= 1M • I
6244 F06b
100k
PD
AT 10kHz
17
6244f