Datasheet

LTC6246/LTC6247/LTC6248
16
624678fa
Figure 3. Single Supply 12-Bit ADC Driver
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 filter. On a single 3.3V supply with a 2.5V
reference, a full –1dBFS output can be obtained without
the amplifier 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.
LTC6246
+
–
624678 F03
LTC2366
V
REF
GND
V
DD
3.3V 2.5V
CS
SDO
SCK
OV
DD
3.3V
V
IN
A
IN
499Ω
1%
499Ω
1%
10pF
Figure 4. 350kHz FFT Showing 82dB SFDR
FREQUENCY (kHz)
0
MAGNITUDE (dB)
0
–10
–30
–50
–70
–20
–40
–60
–80
–90
–100
–110
400 800200 600
624678 F04
1000
f
IN
= 350.195kHz
f
SAMP
= 2.2Msps
SFDR = 82dB
SNR = 70dB
1024 POINT FFT
C3
0.1µF
LTC6246
–
+
624678 F05
3V
LT6003
–
+
3V
3V
3V
V
OUT
= V
R
+ I
PD
• 1M
–3dB BW = 700kHz
I
CC
= 2.2mA
OUTPUT NOISE = 160µV
RMS
MEASURED ON A 1MHz BW
V
OUT
IS REFERRED TO V
R
AT ZERO PHOTOCURRENT, V
OUT
= V
R
V
R
R1
1M, 1%
R3
1k
R2
1k
C2
6.8nF
FILM
OR NPO
Q1
NXP
BF862
PD1
OSRAM
SFH213
I
PD
R5
20k
R4
10k
R6
10M
C1
0.1pF
R7
1k
C4
1µF
Low Noise Low Power DC-Accurate Single Supply
Photodiode Amplifier
Figure 5 shows the LTC6246 applied as a low power high
performance transimpedance amplifier 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
DSS
characteristics. It does
this by placing the JFETs 1mA V
GS
at the gate referenced
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
PD
photocurrent, the output sits at the same volt-
age and rises as photocurrent increases. As mentioned
before, R2 and R3 set the JFET gain to 1 at low frequency.
Figure 5. Low Noise Low Power DC Accurate
Single Supply Photodiode Amplifier