Datasheet

MAX2078
Octal-Channel Ultrasound Front-End
with CW Doppler Mixers
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VGA and CW Mixer Operation
During normal operation, the MAX2078 is configured so
that either the VGA path is enabled while the mixer
array is powered down (VGA mode), or the quadrature
mixer array is enabled while the VGA path is powered
down (CW mode). For VGA mode, set V/C to a logic-
high and for CW mode, set V/C to a logic-low.
Power-Down and Low-Power Mode
The MAX2078 can also be powered down with PD. Set
PD to V
CC1
for power-down mode. In power-down
mode, the device draws a total supply current less than
1µA. Set PD to logic-low for normal operation.
A low-power mode is available to lower the required
power for CWD operation. When selected, the complex
mixers operate at lower quiescent currents and the total
per-channel current is lowered to 34.2mA. Note that
operation in this mode slightly reduces the dynamic
performance of the device. Table 4 shows the logic
function of the standard operating modes.
Applications Information
Mode Select Response Time
The mode select response time is the time that the
device takes to switch between CW and VGA modes.
Figure 2 depicts one possible approach to interfacing
the CW outputs to an instrumentation amplifier, which is
used to drive an ADC. In this implementation, there are
four large-value (in the range of 470nF to 1µF) capaci-
tors between each of the CQ+, CQ-, CI+, CI- outputs
and the circuitry they are driving. The output of the CW
mixer usually drives the input of an instrumentation
amplifier made up of op amps whose input impedance
is set by common-mode setting resistors.
There are clearly both a highpass corner and a lowpass
corner present in this output network. The lowpass cor-
ner is set primarily by the 162Ω mixer pullup resistors,
the series 50Ω resistors, and the shunt 0.022µF capaci-
tor. This lowpass corner is used to filter a combination
of LO leakage and upper sideband. The highpass cor-
ner, however, is of a larger concern since it is dominat-
ed by the combination of a 1µF DC-blocking capacitor
and the pair of shunt 31.6kΩ resistors.
PD
INPUT
V/C CLP VGA
CW
MIXER
INTERNAL
SWITCH
TO VGA
INTERNAL
SWITCH
TO CW
MIXER
3.3V V
CC
CURRENT
CONSUMPTION
5V V
CC
CURRENT
CONSUMPTION
11V V
MIX
CURRENT
CONSUMPTION
1 1 N/A Off Off Off Off 0.3µA 0.4µA 0
1 0 N/A Off Off Off Off 0.1µA 0.6µA 0
0 0 0 Off On Off On 3.2mA 248mA 90.4mA
0 0 1 Off On Off On 3.2mA 216mA 54.4mA
0 1 N/A On Off On Off 88mA 48mA 0
Table 4. Logic Function of Standard Operating Modes
N/A = Not applicable.
162Ω
CI-
CI+
162Ω
1μF
1μF
0.022μF
31.6kΩ
31.6kΩ
50Ω
Figure 2. Typical Example of a CW Mixer’s Output Circuit