LTC6248 Linear Technology Corporation, LTC6248 Datasheet - Page 16

• Manufacturer Part Number: LTC6248
• Description: (LTC6246 - LTC6248) 1mA Power Efficient Rail-to-Rail I/O Op Amps
• Manufacturer: Linear Technology Corporation

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LTC6246/LTC6247/LTC6248
TYPICAL APPLICATIONS
12-Bit ADC Driver
Figure 3 shows the LTC6246 driving an LTC2366 12-bit A/D
converter. The low wideband noise of the LTC6246 main-
tains a 70dB SNR even without the use of an intermediate
antialiasing RC fi lter. On a single 3.3V supply with a 2.5V
reference, a full –1dBFS output can be obtained without
the amplifi er transitioning between input regions, thus
minimizing crossover distortion. Figure 4 shows an FFT
obtained with a sampling rate of 2.2Msps and a 350kHz
input waveform. Spurious free dynamic range is a quite
handsome 82dB.
16
V
IN
499Ω
1%
Figure 4. 350kHz FFT Showing 82dB SFDR
Figure 3. Single Supply 12-Bit ADC Driver
+
–100
–110
–
–10
–20
–30
–40
–50
–60
–70
–80
–90
LTC6246
0
0
3.3V
200
499Ω
10pF
1%
FREQUENCY (kHz)
400
A
600
IN
3.3V 2.5V
V
f
f
SFDR = 82dB
SNR = 70dB
1024 POINT FFT
IN
SAMP
DD
= 350.195kHz
800
LTC2366
V
GND
= 2.2Msps
REF
1000
624678 F04
624678 F03
OV
SDO
SCK
CS
DD
OSRAM
SFH213
Low Noise Low Power DC-Accurate Single Supply
Photodiode Amplifi er
Figure 5 shows the LTC6246 applied as a low power high
performance transimpedance amplifi er for a photodiode.
A low noise JFET Q1 acts as a current buffer, with R2 and
R3 imposing a low frequency gain of approximately 1.
Transimpedance gain is set by feedback resistor R1 to
1MΩ. R4 and R5 set the LTC6246 inputs at 1V below
the 3V rail, with C3 reducing their noise contribution.
By feedback this 1V also appears across R2, setting the
JFET quiescent current at 1mA completely independent
of its pinchoff voltage and I
this by placing the JFETs 1mA V
to the source, which is sitting 1V above ground. For this
JFET, that will typically be about 500mV, and this voltage
is imposed as a reverse voltage on the photodiode PD1.
At zero I
age and rises as photocurrent increases. As mentioned
before, R2 and R3 set the JFET gain to 1 at low frequency.
PD1
I
PD
PD
Figure 5. Low Noise Low Power DC Accurate
Single Supply Photodiode Amplifi er
–3dB BW = 700kHz
I
OUTPUT NOISE = 160μV
V
AT ZERO PHOTOCURRENT, V
CC
photocurrent, the output sits at the same volt-
OUT
= 2.2mA
10M
R6
IS REFERRED TO V
Q1
NXP
BF862
3V
C2
6.8nF
FILM
OR NPO
3V
10k
R4
+
–
R2
1k
R3
1k
LT6003
RMS
R
DSS
20k
3V
MEASURED ON A 1MHz BW
+
–
R5
OUT
1M, 1%
LTC6246
0.1pF
624678 F05
0.1μF
R1
C1
= V
C3
GS
characteristics. It does
R
3V
at the gate referenced
R7
1k
C4
1μF
V
V
OUT
R
= V
R
+ I
PD
624678f
• 1M