Datasheet

16 _____________________________________________________________________________________
MAX4936–MAX4939
Octal High-Voltage Transmit/Receive Switches
Applications Information
For medical ultrasound applications, see Figures 5, 6,
and 7.
Ultrasound-Specific IMD3 Specification
Unlike typical communications applications, the two input
tones are not equal in magnitude for the ultrasound-spe-
cific IMD3 two-tone specification. In this measurement,
F1 represents reflections from tissue and F2 represents
reflections from blood. The latter reflections are typically
25dB lower in magnitude, and hence the measurement
is defined with one input tone 25dB lower than the other.
The IMD3 product of interest (F1 - (F2 - F1)) presents
itself as an undesired Doppler error signal in ultrasound
applications. See Figure 8.
Logic Levels
The digital interface inputs CLK, DIN, LE, and CLR are
tolerant of up to +5.5V, independent of the V
DD
supply
voltage, allowing compatibility with higher voltage con-
trollers.
Daisy-Chaining Multiple Devices
Digital output DOUT is provided to allow the connec-
tion of multiple devices by daisy-chaining (Figure 9).
Connect each DOUT to the DIN of the subsequent
device in the chain. Connect CLK, LE, and CLR inputs of
all devices, and drive LE logic-low to update all devices
simultaneously. Drive CLR high to open all the switches
simultaneously. Additional shift registers can be includ-
ed anywhere in series with the device data chain.
Supply Sequencing and Bypassing
The devices do not require special sequencing of the
V
DD
, V
CC,
and V
EE
supply voltages; however, analog
switch inputs must be unconnected, or satisfy V
EE
P
(V
HV_
, V
COM_
, V
NO_
) P V
CC
during power up and power
down. Bypass V
DD
, V
CC
, and V
EE
to GND with a 1FF
ceramic capacitor as close as possible to the device.
PCB Layout
The pin configuration is optimized to facilitate a very
compact physical layout of the device and its associated
discrete components. A typical application for this device
might incorporate several devices in close proximity to
handle multiple channels of signal processing.
The exposed pad (EP) of the TQFN-EP package provides
a low thermal resistance path to the die. It is important that
the PCB on which the device is mounted be designed to
conduct heat from the EP. In addition, provide the EP with
a low-inductance path to electrical ground. The EP must
be soldered to a ground plane on the PCB, either directly
or through an array of plated through holes.
Figure 5. Ultrasound T/R Path with One Transmit per Receive Channel (One Channel Only)
Application Diagrams
V
EE
GND
V
CC
NO_
HV_
CLK
DIN
DOUT
CLR
LE
COM_
V
DD
RELAY
MUX
CONNECTORS
HV
MUX
TRANSDUCERS
SPI
CONTROL
-5V
XMT
RCV
+5V+3V
HV
MUX
HV
MUX
MAX4936/MAX4938