e7804 Semtech Corporation, e7804 Datasheet - Page 6

• Manufacturer Part Number: e7804
• Description: 133 Mhz Quad Pin Electronics Driver And Window Comparator
• Manufacturer: Semtech Corporation

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Introduction
The four driver and window comparator channels of the
E7804 are shown in Figure .
Driver
Refer to Table  showing the modes of operation of the
driver.
Each channel’s driver states of HiZ, force DVH voltage, force
DVL voltage and Open can be controlled either by external
input pins or via internal registers and bits programmed
through the serial interface.
The HiZ/DVH/DVL states are controlled by the differential,
external inputs, DHI/DHI* and DEN/DEN*. Each channel
also has internal register bits (see Table 2, CH[0:3]_Relays_
&_States registers) SDHI and SDEN that accomplish the
same functions as DHI and DEN. The serial register has
another bit, SEN (Serial Enable) that allows the SDI and
SDEN bits to override the external input pins DHI and
DEN. If SEN is a logical “0”, the SDHI and SDEN bits are
ignored.
The DHI/DHI* and DEN/DEN* inputs are LV_TTL and
differential LVDS, LV_PECL compatible.
Unused DHI/DHI* or DEN/DEN* must be tied to valid
logic levels.
Optimizing Driver Waveforms
The driver output pin, VIO, will normally be connected to the
parametric output pin, POUT, when designed into a system.
See the recommended 7804 Hookup drawing farther on
in this datasheet. The POUT pin has a lumped capacitance
associated with it that will degrade the signal integrity of
the driver output waveform if not properly compensated
for. The recommendation is to insert ferrite devices
between the connection of POUT to VIO to accomplish this.
For more details on how and why this approach is used,
please read Semtech Application Note #ATE-A3 ATE-to-DUT
Interface: Using Ferrites to Replace Relays for Lower Cost
and Improved Performance.
The driver output impedance has a reactive component
to it and will not completely absorb reflections from an
TEST AND MEASUREMENT PRODUCTS
Circuit Description (continued)
2007 Semtech Corp. / Rev. 7, 02/26/07
6
unterminated transmission line. This is common for all
drivers, but more so in CMOS drivers than high speed
bipolar stages. The figure here shows how transmission
line length, here in the form of coaxial cable, will sum the
reflections constructively and destructively to alter the
peak-to-peak waveform excursions across frequency. More
data on this performance and methods for optimizing signal
integrity and extending Fmax will be available from Semtech
staff. Check with Semtech for the latest information. From
the graph below one can see that E7804 driver signals in
excess of 50MHz are possible.
Driver Levels
Each channel’s DVH and DVL are high input impedance
voltage inputs which establish the channel driver’s high
and low levels. The driver’s output range is –0.2V, +5.2V.
DVH to VIO and DVL to VIO offset errors are small, which
allow the E7804 to be configured with common input levels
to each channel (i.e. DVH[0:3] may be connected together
externally, and the same for DVL[0:3]).
Driver Source Impedance
Drivers feature a self-calibrating source impedance
calibrated to match an external resistor, REREF, connected
between the EREF pin and analog ground. The source
impedance can be chosen to calibrate in the range 48W
to 0W using the calculation REREF/00.
A driver’s source impedance is affected by its DVH and
DVL levels, and therefore needs recalibrating whenever
driver levels into the chip are changed. The calibration
routine is initiated via the serial interface (see Table 2,
calibrate_output_z register). When initiated, all channels
are recalibrated in parallel.
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E7804