Datasheet

LT6350
25
6350fc
For more information www.linear.com/LT6350
+
–
+
IN1
–
IN1
V
+
V
–
OUT2
OUT1
SHDN
+
IN2
5V
2.5V
499Ω
8 7 3 5
1 2 6 4
0.1µF
0.1µF
2.5V
1000pF
R3
523Ω
LT6350
6350 F13
+
–
V
OUT1
V
OUT2
C5
1000pF
4750Ω
0.01µF
V
IN
+
–
174Ω 2210Ω
Figure 13. 100KHz, 3rd Order Butterworth Filter
applicaTions inForMaTion
100kHz, 3RD-ORDER BUTTERWORTH FILTER
The LT6350 can be configured as a single-ended to
differential filter incorporating feedback from the inverting
output. Figure 13 shows the schematic of the configuration
with values giving a 3rd Order Butterworth characteristic
having a 100kHz –3dB point with a differential gain of four.
Figure 14 shows the filter output response to 10MHz. As
an option, to match the source impedance and preserve
the low DC errors of the LT6350, connect a 2.10k series
resistor at +IN1. To reduce the resistor noise, the +IN1
pin can be bypassed with a 0.1µF capacitor. For similar
topologies please consult the LT1567 data sheet and
design guide.
Low Noise, Low Power 1MΩ Single Supply Photo-
diode Differential Output Transimpedance Amplifier
The Typical Application on the back page shows the LT6350
applied as a differential output transimpedance amplifier.
The LT6350 forces the BF862 ultralow noise JFET source
to 3V, with R2 ensuring that the JFET has an I
DRAIN
of 1mA.
The JFET acts as a source follower, buffering the input of
the LT6350 and making it suitable for the high impedance
feedback element R1. The BF862 has a minimum I
DSS
of
10mA and a pinchoff voltage between –0.3V and –1.2V.
The JFET gate and OUT1 therefore sit at a point slightly
higher than one pinchoff voltage below 3V, about mid-
supply at 2.5V.
When the photodiode is illuminated, the current must come
from OUT1 through R1 as in a normal transimpedance
amplifier. Amplifier output noise density is dominated at
low frequency by the 130nV/√Hz of the feedback resistor,
rising to 210nV/√Hz at 1MHz. Note that because the JFET
has a high g
m
, approximately 1/30Ω, its attenuation looking
into R2 is only about 1%. The closed-loop bandwidth
using a 3pF photodiode was measured at approximately
1.35MHz. With the output taken differentially, the gain and
the noise are both doubled.
Figure 14. 100KHz, 3rd Order Butterworth Filter Response
FREQUENCY (Hz)
1K
GAIN (dB)
20
–60
–40
–80
–20
0
–100
100K 1M
6350 F14
10M10K