LT6220/LT6221/LT6222

16

sn622012 622012fs

PACKAGE DESCRIPTIO

As the signal level increases though, the output of the op

amp goes more negative. At 12.5µA of photodiode cur-

rent, the 100kΩ 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

circuitry. 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

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:

Figure 4. 3V, 1MHz, 4th Order Butterworth Filter

Figure 5. Frequency Response of Filter

Figure 3. Stepped-Gain Photodiode Amplifier Response

PHOTO

CURRENT

100µA/DIV

0.5V/DIV

5µs/DIV

622012 F03

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

909Ω

2.67k

909Ω

1/2 LT6221

22pF

470pF

622012 F04

1.1k

2.21k

3V

1.1k

FREQUENCY (Hz)

–80

–40

0

–100

–60

–20

20

1k

100k

1M

10M

100M

622012 F05

–120

10k