LT6201 datasheet(18/26 Pages) AD

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LTC6246/LTC6247/LTC6248

Rev C

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Thisisnotthelowestnoiseconfigurationforatransistor,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

theoutputnoiseisdominatedbythe130nV/√Hzofthe1MΩ

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

integratedoutputnoisebeing160µVRMSupto1MHz.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 2VP-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

2.5V

–2.5V

2.5V

–2.5V

VIN

1/2LTC6247

1.5k

1/2LTC6247

660nF

VOUT

30k

Figure 6. 60dB 5.5MHz Gain Block

FREQUENCY (kHz)

10k

100k

10M

624678 F07

VS = ±2.5V

VIN = 4.5mVP-P

RL = 1k

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

VIN

1.1k

2.3k

1/2LTC6247

12pF

2.7k

2.7V

1.2V

910

1/2LTC6247

120pF

2.7V

VOUT

5.6pF

1.1k

FREQUENCY (kHz)

10k

–10

–30

–50

–70

–20

–40

–60

–80

–90

–100

100k

10M

624678 F09

100M

VS = 2.7V, 0V

VIN = 2VP-P

RL = 1kΩ to 0V

Figure 9

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