ltc6244hv Linear Technology Corporation, ltc6244hv Datasheet - Page 18

• Manufacturer Part Number: ltc6244hv
• Description: Dual 50mhz, Low Noise, Rail-to-rail, Cmos Op Amp
• Manufacturer: Linear Technology Corporation

LTC6244
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
APPLICATIO S I FOR ATIO
18
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
the Photodiode Looks Into a Noninverting Input. Pin 5 was Selected Because it is in the Corner, Removing One Lead Capacitance
PHOTODIODE
SMALL AREA
C
TEMD1000
PD
Figure 7. LTC6244 in a Normal TIA Confi guration
VISHAY
= 1.8pF
–5V
DM
I
U
PD
and one C
PHOTODIODE
SMALL AREA
C
U
TEMD1000
–
LTC6244HV
+
PD
DM
VISHAY
= 1.8pF
1/2
Accross A1’s Inputs, Reducing Amplifi er Input Capacitance. Inversion is Provided by A2, so that
–5V
5V
CM
0.1pF
6244 F07
1M
(because the +input is
C
R
F
F
W
–5V
I
PD
V
BW = 350kHz
NOISE = 120 V
MEASURED ON A
350kHz BW
OUT
V
OUT
5
6
= 1M • I
R1
499
–
+
LTC6244HV
A1 1/2
U
PD
5V
RMS
(PARASITIC)
8
0.07pF
56pF
1M
R2
1k
C1
R
F
7
R3
1k
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
measured over that bandwidth.
The circuit of Figure 8a makes some slight improvements.
Operation is still transimpedance mode, with R
the gain to 1MΩ. 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
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.
DM
2
3
(3.5pF), and leaving only one C
–
LTC6244HV
+
A2 1/2
6.98k
R4
–5V
4
150pF
C2
1
6244 F08a
V
BW = 1.6MHz
NOISE = 1.2mV
MEASURED ON A
2MHz BW
OUT
V
= 1M • I
OUT
PD
RMS
CM
(2.1pF). The op
F
setting
RMS
6244f