Datasheet

LTC6246/LTC6247/LTC6248
17
624678fa
This is not the lowest noise configuration for a transistor, as
downstream noise sources appear at the input completely
unattenuated. At low frequency, this is not a concern for a
transimpedance amplifier because the noise gain is 1 and
the output noise is dominated by the 130nV/√Hz of the 1MΩ
R1. However, at increasing frequencies the capacitance
of the photodiode comes into play and the circuit noise
gain rises as the 1MΩ feedback looks back into lower and
lower impedance. But capacitor C2 comes to the rescue.
In addition to the obvious quenching of noise source R3,
capacitor C2 increases the JFET gain to about 30 at high
frequency effectively attenuating the downstream noise
contributions of R2 and the op amp input noise. Thus the
circuit achieves low input voltage noise at high frequency
where it is most needed. Amplifier LT6003 is used to
buffer the output voltage of the photodiode and R7 and
C4 are used to filter out the voltage noise of the LT6003.
Bandwidth to 700kHz was achieved with this circuit, with
integrated output noise being 160µV
RMS
up to 1MHz. Total
supply current was a very low 2.2mA.
Typical applicaTions
60dB 5.5MHz Gain Block
Figure 6 shows the LTC6247 configured as a low power
high gain high bandwidth block. Two amplifiers each
configured with a gain of 31V/V, are cascaded in series. A
660nF capacitor is used to limit the DC gain of the block
to around 30dB to minimize output offset voltage. Figure 7
shows the frequency response of the block. Mid-band
voltage gain is approximately 60dB with a –3dB frequency
of 5.5MHz, thus resulting in a gain-bandwidth product of
5.5GHz with only 1.9mA of quiescent supply current.
Single 2.7V Supply 4MHz 4th Order Butterworth Filter
Benefitting from low voltage operation and rail-to-rail
output, a low power filter that is suitable for antialiasing
can be built as shown in Figure 8. On a 2.7V supply the
filter has a passband of approximately 4MHz with 2V
P-P
input signal and a stopband attenuation that is greater than
–75dB at 43MHz as shown in Figure 9. The resistor and
capacitor values can be scaled to reduce noise at the cost
of large signal power consumption and distortion.
624678 F06
–
+
1k
2.5V
–2.5V
2.5V
–2.5V
V
IN
1/2LTC6247
50Ω
1.5k
–
+
1/2LTC6247
660nF
V
OUT
30k
Figure 6. 60dB 5.5MHz Gain Block
FREQUENCY (kHz)
10k
GAIN (dB)
65
60
50
40
30
55
45
35
25
20
1M100k 10M
624678 F07
V
S
= ±2.5V
V
IN
= 4.5mV
P-P
R
L
= 1kΩ
DC GAIN = 30dB
(DUE TO 660nF DC BLOCKING CAP)
OUTPUT OFFSET = 4mV
Figure 7
Figure 8. Single 2.7V Supply 4MHz
4th Order Butterworth Filter
624678 F08
56pF
–
+
V
IN
1.1k 2.3k
1/2LTC6247
12pF
2.7k
2.7V
1.2V
910Ω
910Ω
–
+
1/2LTC6247
120pF
2.7V
V
OUT
5.6pF
1.1k
FREQUENCY (kHz)
10k
GAIN (dB)
10
–10
–30
–50
–70
–20
–40
–60
–80
–90
0
–100
100k 10M1M
624678 F09
100M
V
S
= 2.7V, 0V
V
IN
= 2V
P-P
R
L
= 1kΩ to 0V
Figure 9