LT6220/LT6221/LT6222
16
622012fc
For more information www.linear.com/LT6220/LT6221/LT6222
Typical applicaTions
As the signal level increases though, the output of the op
amp goes more negative. At 12.5µA of photodiode cur-
rent, the
100
kΩ gain dictates that the LT6220 output will
be about 1.25V below ground. However, at that point the
emitter of Q2 will be at ground, and the base of Q1 will
be 1V below ground. Thus, Q1 turns on and photodiode
current starts to flow through R2. The transimpedance gain
is therefore now reduced to R1||R2, or about 3.1kΩ. The
circuit response is shown in Figure 3. Note the smooth
transition between the two operating gains, as well as
the linearity.
Differential-In/Differential-Out Amplifier
The circuit of Figure 6 shows the LT6222 applied as a
buffered differential-in differential-out amplifier with a gain
of 2. Op amps A and B are configured as simple unity-gain
buffers, offering high input impedance to upstream cir
-
cuitry. Resistors
R1 and R2 perform an averaging function
on the common mode input voltage and R3 attenuates it
by a factor of 2/3 and references it to the voltage source
V
OCM
. The resultant voltage, V
MID
= 2/3 • V
ICM
, is placed
at the noninverting inputs of op amps C and D. The other
four
resistors set gains of +3 from the noninverting input
and –2 through the inverting path. Thus the output voltage
of the upper path is:
–OUT = 3 •(2/3 • V
ICM
+ 1/3 • V
OCM
) – 2
• (V
ICM
+ V
DIFF
/2)
= 2V
ICM
+ V
OCM
– 2V
ICM
– V
DIFF
= V
OCM
– V
DIFF
Figure 4. 3V, 1MHz, 4th Order Butterworth Filter
Figure 5. Frequency Response of Filter
Figure 3. Stepped-Gain Photodiode Amplifier Response
Single 3V Supply, 1MHz, 4th Order Butterworth Filter
The circuit shown in Figure 4 makes use of the low voltage
operation and the wide bandwidth of the LT6221 to create
a DC accurate 1MHz 4th order lowpass filter powered from
a 3V supply. The amplifiers are configured in the inverting
mode for the lowest distortion and the output can swing
rail-to-rail for maximum dynamic range. Figure 5 displays
the frequency response of the filter. Stopband attenuation
is greater than 100dB at 50MHz.
–
+
1/2 LT6221
47pF
220pF
V
IN
S
/2
909Ω
2.67k
909Ω
–
+
1/2 LT6221
22pF
V
470pF
1.1k
2.21k
3V
1.1k
FREQUENCY (Hz)
–80
–40
0
–100
–60
–20
1k 100k 1M 10M
–120
10k
PHOTO
CURRENT
V
OUT
0.5V/DIV
