Datasheet

LTC6244
19
6244fb
Small Area Photodiode Amplifi ers
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 amplifi er 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 amplifi er using the LTC6244
is shown in Figure 7. The input capacitance of the ampli-
fi er consists of C
DM
and one C
CM
(because the +input is
Figure 8a: Using Both Op Amps for Higher Bandwidth. A1 Provides a Gain of 3 Within the Loop, Increasing the Gain Bandwidth
Product. This Bootstraps the C
DM
Accross A1’s Inputs, Reducing Amplifi er Input Capacitance. Inversion is Provided by A2, so that
the Photodiode Looks Into a Noninverting Input. Pin 5 was Selected Because it is in the Corner, Removing One Lead Capacitance
Figure 7. LTC6244 in a Normal TIA Confi guration
grounded), or about 6pF total. The small photodiode has
1.8pF, so the input capacitance of the amplifi er 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
bandwidth of 350kHz, with an output noise of 120µV
RMS
measured over that bandwidth.
The circuit of Figure 8a makes some slight improvements.
Operation is still transimpedance mode, with R
F
setting
the gain to 1M. 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 amplifi er input capacitance is bootstrapped by the
feedback of R2:R1, eliminating the effect of A1’s input
C
DM
(3.5pF), and leaving only one C
CM
(2.1pF). The op
amp at Pins 5, 6 and 7 was chosen for the input amplifi er
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 confi gura-
tion, input capacitance is minimized.
APPLICATIONS INFORMATION
5V
V
OUT
= 1M • I
PD
BW = 350kHz
NOISE = 120µV
RMS
MEASURED ON A
350kHz BW
I
PD
R
F
1M
SMALL AREA
PHOTODIODE
VISHAY
TEMD1000
C
PD
= 1.8pF
C
F
0.1pF
–5V
–5V
6244 F07
–
+
1/2
LTC6244HV
V
OUT
5V
–5V
I
PD
R
F
1M
R3
1k
R2
1k
R4
6.98k
R1
499Ω
SMALL AREA
PHOTODIODE
VISHAY
TEMD1000
C
PD
= 1.8pF
0.07pF
(PARASITIC)
C2
150pF
C1
56pF
–5V
6244 F08a
–
+
A1 1/2
LTC6244HV
V
OUT
8
4
1
7
5
6
–
+
A2 1/2
LTC6244HV
2
3
V
OUT
= 1M • I
PD
BW = 1.6MHz
NOISE = 1.2mV
RMS
MEASURED ON A
2MHz BW