Datasheet

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INTERFACING TO ADCs
3
0
3
6
9
12
15
18
21
-
-
-
-
-
-
-
100
NormalizedAmplitude(dB)
Frequency(MHz)
1
10
10MHz
15MHz
CW DOPPLER PROCESSING
ADS5281
12-Bit
40MSPS
IN
P
IN
N
VCA8500
OUT
OUT
Optional
LPF
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
a minimum (C
LOAD
≤ 18pF, differential). The user uses sampling rates of up to 40MSPS. Here, the ratio
should examine all factors that contribute to the total of the bandwidth (BW) to the Nyquist frequency (f
S
/2)
load (R
LTOTAL
= R
L
+ X
L
). Depending on the overall is approximately 0.75, which provides a good
system requirements, trade-offs can be made compromise between the passband area and the stop
between the output loading and the desired distortion band attenuation. Choosing the lower 10MHz
levels and output swing. bandwidth setting may be considered if the sampling
rate is reduced further, or if the input signal
bandwidth is lower. In this case, the reduced noise
bandwidth can potentially improve the noise floor.
The VCA8500 is ideally suited to drive the ADS5281 ,
a low-power, octal, 12-bit ADC that can be operated
at sampling rates of up to 50MSPS. The VCA outputs
can be directly connected the ADC inputs without the
need for any external components, as shown in
Figure 67 . Observing proper layout considerations,
the two devices can be placed in close proximity to
each other and allow for a very compact printed
circuit board (PCB) layout.
The ADS5281 features many performance
characteristics that make it an excellent choice for
ultrasound systems: low channel power of only
55mW/ch (at 40MSPS); high signal-to-noise ratio of
70dB; and fast overload recovery time of only one
clock cycle. The VCA8500 can be configured to
complement this level of performance by choosing
the most suitable amplification setting of the post-gain
Figure 68. Normalized Frequency Response of the
amplifier. For example, the ADS5281 has a full-scale
10MHz and 15MHz Low-Pass Filter
input of 2V
PP
and an input-referred noise of
approximately 50nV/ √ Hz. In order to achieve the
highest combined dynamic range performance, the
PGA gain can be set to 20dB. With this gain setting,
The VCA8500 integrates many of the elements
the output-referred noise is dominated by the noise
necessary to allow for the implementation of a CW
contribution of the attenuator and PGA and remains
doppler processing circuit, such as a V/I converter for
constant over most of the gain control range
each channel and a cross-point switch matrix with an
(approximately 65nV/ √ Hz). Only at the high end of
8-input into 10-output (8 × 10) configuration.
the gain control range does the LNA and
source-related noise contribution become the
In order to switch the VCA8500 from the default TGC
prevailing factor. Higher gain PGA settings may be
mode operation into CW mode, bit D5 of the control
chosen to interface to lower resolution ADCs that
register must be updated to low ('0'). This setting also
have a higher noise floor.
enables access to all other registers that determine
the switch matrix configuration (see the Input Register
Bit Map tables). In order to process CW signals, the
LNA internally feeds into a differential V/I amplifier
stage. The transconductance of the V/I amplifier is
typically 16.4mA/V with a 100mV
PP
input signal. For
proper operation, the CW outputs must be connected
to an external bias voltage of +2.5V. Each CW output
Figure 67. The VCA8500 Can Be Interfaced to the
is designed to sink a small dc current of 0.9mA, and
ADS5281 Without the Need for External
can deliver a signal current up to 2.9mA
PP
.
Components
Figure 68 shows the normalized frequency response
of the low-pass filter. The 15MHz bandwidth is
intended to be the upper bandwidth for a system that
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