AD8139 AD [Analog Devices], AD8139 Datasheet - Page 22

• Manufacturer Part Number: AD8139
• Description: Low Noise Rail-to-Rail Differential ADC Driver
• Manufacturer: AD [Analog Devices]

AD8139
Driving a Capacitive Load
A purely capacitive load will react with the bondwire and pin
inductance of the AD8139, resulting in high frequency ringing
in the transient response and loss of phase margin. One way to
minimize this effect is to place a small resistor in series with
each output to buffer the load capacitance, see Figure 6 and
Figure 61. The resistor and load capacitance will form a first-
order low-pass filter; therefore, the resistor value should be as
small as possible. In some cases, the ADCs require small series
resistors to be added on their inputs.
The Typical Performance Characteristics that illustrate transient
response versus the capacitive load were generated using series
resistors in each output and a differential capacitive load.
Layout Considerations
Standard high speed PCB layout practices should be adhered to
when designing with the AD8139. A solid ground plane is recom-
mended and good wideband power supply decoupling networks
should be placed as close as possible to the supply pins.
To minimize stray capacitance at the summing nodes, the
copper in all layers under all traces and pads that connect to the
summing nodes should be removed. Small amounts of stray
summing-node capacitance will cause peaking in the frequency
response, and large amounts can cause instability. If some stray
summing-node capacitance is unavoidable, its effects can be
compensated for by placing small capacitors across the feedback
resistors.
Terminating a Single-Ended Input
Controlled impedance interconnections are used in most high
speed signal applications, and they require at least one line
termination. In analog applications, a matched resistive
termination is generally placed at the load end of the line. This
section deals with how to properly terminate a single-ended
input to the AD8139.
–10
–11
–12
–13
–1
–2
–3
–4
–5
–6
–7
–8
–9
5
4
3
2
1
0
10M
V
V
G = 1 (R
R
S
O, dm
L, dm
=
±
Various Capacitive Load and Series Resistance
5V
= 1kΩ
= 0.1V p-p
F
= R
Figure 61. Frequency Response for
G
= 200Ω)
FREQUENCY (MHz)
R
C
S
L
= 15pF
= 30.1Ω
R
C
S
L
100M
= 15pF
= 60.4Ω
R
S
C
= 60.4Ω
L
= 5pF
R
C
S
L
R
C
= 30.1Ω
= 5pF
S
L, dm
= 0Ω
= 0pF
1G
Rev. A | Page 22 of 24
The input resistance presented by the AD8139 input circuitry is
seen in parallel with the termination resistor, and its loading
effect must be taken into account. The Thevenin equivalent
circuit of the driver, its source resistance, and the termination
resistance must all be included in the calculation as well. An
exact solution to the problem requires the solution of several
simultaneous algebraic equations and is beyond the scope of
this data sheet. An iterative solution is also possible and simpler,
especially considering the fact that standard 1% resistor values
are generally used.
Figure 62 shows the AD8139 in a unity-gain configuration
driving the AD6645, which is a 14-bit high speed ADC, and
with the following discussion, provides a good example of how
to provide a proper termination in a 50 Ω environment.
The termination resistor, R
resistance of the AD8139 circuit (calculated using Equation 19),
yields an overall input resistance of 50 Ω that is seen by the
signal source. In order to have matched feedback loops, each
loop must have the same R
input (upper) loop, R
with the (+) input plus the parallel combination of R
source resistance of 50 Ω. In the upper loop, R
equal to 228 Ω. The closest standard 1% value to 228 Ω is 226 Ω
and is used for R
achieved by using two resistors in series to obtain a resistance
closer to 228 Ω.
Things get more complicated when it comes to determining the
feedback resistor values. The amplitude of the signal source
generator V
when terminated in 50 Ω. Thus, a 2 V p-p terminated amplitude
is produced by a 4 V p-p amplitude from V
equivalent circuit of the signal source and R
when calculating the closed-loop gain because in the upper loop
R
tance looking back toward the source. The Thevenin voltage of
the signal source is greater than the signal source output voltage
when terminated in 50 Ω because R
than 50 Ω. In this case, it is 61.9 Ω and the Thevenin voltage
and resistance are 2.2 V p-p and 28 Ω, respectively. Now the
upper input branch can be viewed as a 2.2 V p-p source in series
with 228 Ω. Since this is a unity-gain application, a 2 V p-p
differential output is required, and R
(2/2.2) = 206 Ω. The closest standard value to this is 205 Ω.
When generating the Typical Performance Characteristics data,
the measurements were calibrated to take the effects of the
terminations on closed-loop gain into account.
G
is split between the 200 Ω resistor and the Thevenin resis-
S
is two times the amplitude of its output signal
G
in the lower loop. Greater accuracy could be
G
is equal to the 200 Ω resistor in series
T
G
, in parallel with the 268 Ω input
if they have the same R
T
F
must always be greater
must therefore be 228 ×
S
. The Thevenin
T
must be used
G
is therefore
F
. In the
T
and the