参数资料
型号: LTC6244CMS8#PBF
厂商: Linear Technology
文件页数: 11/26页
文件大小: 0K
描述: IC OP AMP DUAL R-R 8-MSOP
标准包装: 50
放大器类型: 通用
电路数: 2
输出类型: 满摆幅
转换速率: 35 V/µs
增益带宽积: 50MHz
电流 - 输入偏压: 1pA
电压 - 输入偏移: 40µV
电流 - 电源: 6.25mA
电流 - 输出 / 通道: 35mA
电压 - 电源,单路/双路(±): 2.8 V ~ 6 V
工作温度: 0°C ~ 70°C
安装类型: 表面贴装
封装/外壳: 8-TSSOP,8-MSOP(0.118",3.00mm 宽)
供应商设备封装: 8-MSOP
包装: 管件
产品目录页面: 1322 (CN2011-ZH PDF)
LTC6244
19
6244fb
Small Area Photodiode Ampliers
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 amplier 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 amplier using the LTC6244
is shown in Figure 7. The input capacitance of the ampli-
er consists of CDM and one CCM (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 CDM Accross A1’s Inputs, Reducing Amplier 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 Conguration
grounded), or about 6pF total. The small photodiode has
1.8pF, so the input capacitance of the amplier 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 120μVRMS
measured over that bandwidth.
The circuit of Figure 8a makes some slight improvements.
Operation is still transimpedance mode, with RF setting
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 amplier input capacitance is bootstrapped by the
feedback of R2:R1, eliminating the effect of A1’s input
CDM (3.5pF), and leaving only one CCM (2.1pF). The op
amp at Pins 5, 6 and 7 was chosen for the input amplier
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 congura-
tion, input capacitance is minimized.
APPLICATIONS INFORMATION
5V
VOUT = 1M IPD
BW = 350kHz
NOISE = 120μVRMS
MEASURED ON A
350kHz BW
IPD
RF
1M
SMALL AREA
PHOTODIODE
VISHAY
TEMD1000
CPD = 1.8pF
CF
0.1pF
–5V
6244 F07
+
1/2
LTC6244HV
VOUT
5V
–5V
IPD
RF
1M
R3
1k
R2
1k
R4
6.98k
R1
499Ω
SMALL AREA
PHOTODIODE
VISHAY
TEMD1000
CPD = 1.8pF
0.07pF
(PARASITIC)
C2
150pF
C1
56pF
–5V
6244 F08a
+
A1 1/2
LTC6244HV
VOUT
8
4
1
7
5
6
+
A2 1/2
LTC6244HV
2
3
VOUT = 1M IPD
BW = 1.6MHz
NOISE = 1.2mVRMS
MEASURED ON A
2MHz BW
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