LTC6244

18

6244f

Small Area Photodiode Amplifiers

Small area photodiodes have very low capacitance, typically

under 10pF and some even below 1pF. Their low capaci-

tance makes them more approximate current sources to

higher frequencies than large area photodiodes. One of

the challenges of small area photodiode amplifier design

is to maintain low input capacitance so that voltage noise

does not become an issue and current noise dominates. A

simple small area photodiode amplifier using the LTC6244

is shown in Figure 7. The input capacitance of the ampli-

APPLICATIO S I FOR ATIO

Figure 8a: Using Both Op Amps for Higher Bandwidth. A1 Provides a Gain of 3 Within the Loop, Increasing the Gain Bandwidth

the Photodiode Looks Into a Noninverting Input. Pin 5 was Selected Because it is in the Corner, Removing One Lead Capacitance

5V

–5V

1M

R3

1k

R2

1k

R4

6.98k

R1

499

9

SMALL AREA

PHOTODIODE

VISHAY

TEMD1000

0.07pF

(PARASITIC)

C2

150pF

C1

56pF

–5V

6244 F08a

–

+

A1 1/2

LTC6244HV

8

4

1

7

5

6

–

+

A2 1/2

LTC6244HV

2

3

BW = 1.6MHz

MEASURED ON A

2MHz BW

5V

BW = 350kHz

NOISE = 120

MEASURED ON A

350kHz BW

1M

SMALL AREA

PHOTODIODE

VISHAY

TEMD1000

0.1pF

–5V

–5V

6244 F07

–

+

1/2

LTC6244HV

Figure 7. LTC6244 in a Normal TIA Configuration

grounded), or about 6pF total. The small photodiode has

1.8pF, so the input capacitance of the amplifier is dominating

the capacitance. The small feedback capacitor is an actual

component (AVX Accu-F series), but it is also in parallel

with the op amp lead, resistor and parasitic capacitances,

so the total real feedback capacitance is probably about

0.4pF. The reason this is important is that this sets the

compensation of the circuit and, with op amp gain band-

width, the circuit bandwidth. The circuit as shown has a

measured over that bandwidth.

The circuit of Figure 8a makes some slight improvements.

the gain to 1M

Ω. However, a noninverting input stage A1

with a gain of 3 has been inserted, followed by the usual

inverting stage performed by A2. Note what this achieves.

The amplifier input capacitance is bootstrapped by the

feedback of R2:R1, eliminating the effect of A1’s input

amp at Pins 5, 6 and 7 was chosen for the input amplifier

to eliminate extra pin-to-pin capacitance on the (+) input.

The lead capacitance on the corner of an MSOP package is

only about 0.15pF. By using this noninverting configura-

tion, input capacitance is minimized.