AD745JR-16-REEL7 Analog Devices Inc, AD745JR-16-REEL7 Datasheet - Page 8

• Manufacturer Part Number: AD745JR-16-REEL7
• Description: IC OPAMP BIFET 20MHZ ULN 16SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD745JR-16-REEL7

• Rohs Status: RoHS non-compliant
• Amplifier Type: J-FET
• Number Of Circuits: 1
• Slew Rate: 12.5 V/µs
• Gain Bandwidth Product: 20MHz
• -3db Bandwidth: 20MHz
• Current - Input Bias: 150pA
• Voltage - Input Offset: 250µV
• Current - Supply: 8mA
• Current - Output / Channel: 40mA
• Voltage - Supply, Single/dual (±): 9.6 V ~ 36 V, ±4.8 V ~ 18 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.300", 7.5mm Width)
• Output Type: -

AD745
Figures 5 and 6 show two ways to buffer and amplify the output
of a charge output transducer. Both require the use of an ampli-
fier that has a very high input impedance, such as the AD745.
Figure 5 shows a model of a charge amplifier circuit. Here,
amplification depends on the principle of conservation of charge
at the input of amplifier A1, which requires that the charge on
capacitor C
output voltage of ∆Q/C
appear at the output amplified by the noise gain (1 + (C
of the circuit.
The second circuit, Figure 6, is simply a high impedance fol-
lower with gain. Here the noise gain (1 + (R1/R2)) is the same
as the gain from the transducer to the output. Resistor R
both circuits, is required as a dc bias current return.
There are three important sources of noise in these circuits.
Amplifiers A1 and A2 contribute both voltage and current noise,
while resistor R
where:
This must be root-sum-squared with the amplifier’s own current
noise.
Figure 5 shows that these two circuits have an identical frequency
response and the same noise performance (provided that
C
network is used to increase the effective resistance of R
improve the low frequency cutoff point by the same factor.
S
k = Boltzman’s Constant = 1.381 × 10
T = Absolute Temperature, Kelvin (0°C = 273.2 Kelvin)
∆ f = Bandwidth – in Hz (Assuming an Ideal “Brick Wall”
N
/C
~
F
=
= R1/ R2). One feature of the first circuit is that a “T”
Filter)
4
k
S
be transferred to capacitor C
R
T
B
B
∆
contributes a current noise of:
f
R2
C
S
F
. The amplifiers input voltage noise will
C
C
C
S
B
R
B
B
OPTIONAL, SEE TEXT.
R
R
B
B
R
S
C
R1
A2
A1
F
R2
R1
R2
R1
=
–23
C
C
F
S
F
, thus yielding an
Joules/Kelvin
B
S
B
and
/C
, in
F
))
However, this does not change the noise contribution of R
which, in this example, dominates at low frequencies. The graph
of Figure 8 shows how to select an R
this resistor’s contribution to overall circuit noise. When the
equivalent current noise of R
To maximize dc performance over temperature, the source
resistances should be balanced on each input of the amplifier.
This is represented by the optional resistor R
As previously mentioned, for best noise performance care should
be taken to also balance the source capacitance designated by
C
Figure 6. At values of C
impact on noise; capacitor C
bypass capacitor of 0.01 µF or greater.
I
B
B
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
–210
–220
(
The value for C
5.2
2qI
0.01
5.2
5.2
5.2
5.2
B
10
10
10
10
10
)
10
9
8
7
6
1pA
, there is diminishing return in making R
0.1
1
B
I
in Figure 5 would be equal to C
10pA
B
FREQUENCY – Hz
(
B
10
over 300 pF, there is a diminishing
2qI
INPUT BIAS CURRENT
B
B
can then be simply a large mylar
(( 4 kT)/R) equals the noise of
B
100
)
4 kT/R
100pA
B
large enough to minimize
1k
B
Ω
10k
in Figures 5 and 6.
1nA
100k
B
TOTAL
OUTPUT
NOISE
NOISE DUE TO
R
NOISE DUE TO
I
B
larger.
B
S
ALONE
ALONE
in
10nA
B