LTM9012_15 Datasheet, PDF (14/28 Pages) Linear Technology – Quad 14-Bit, 125Msps ADC with Integrated Drivers

LTM9012
Applications Information
Converter Operation
The LTM9012 is a low power, 4-channel, 14-bit, 125Msps
A/D converter that is powered by a 1.8V ADC supply and
3.3V driver supplies. Each input includes a fixed gain,
differential amplifier. The analog inputs can be driven dif-
ferentially or single-ended. The encode input can be driven
differentially for optimal jitter performance, or single-ended
for lower power consumption. The digital outputs are serial
LVDS to minimize the number of data lines. Each channel
outputs two bits at a time (2-lane mode). At lower sampling
rates there is a one bit per channel option (1-lane mode).
Many additional features can be chosen by programming
the mode control registers through a serial SPI port.
Analog Inputs
The analog inputs for each channel of the LTM9012 con-
sist of a differential amplifier with fixed gain followed by
a lowpass filter. The 10x gain version has 49.9Ω series
resistance in each input.
The differential input can support single-ended operation
by connecting the inverting input to a fixed DC voltage or
ground. However, if ground is used, there will be a 6dB
loss of dynamic range. For maximum dynamic range, con-
nect the inverting inputs of the LTM9012 to a DC voltage
equal to the median of the voltage excursions of the non-
inverting input. An op amp provides an excellent means
of providing a low impedance voltage source capable of
sourcing and sinking small amounts of current. Note the
value of this DC voltage should fall between the limits of
allowable input common mode voltages. See Figure 2 for
an example.
LTM9012
(1 CHANNEL SHOWN)
SIGNAL
–
+
¼ LTC6254
V_REF
–
0.1µF ⁺
R_F
9012 F02
SET V_REF EQUAL TO THE DC MEDIAN OF THE SIGNAL VOLTAGE
Figure 2. Single-Ended Operation
The gain of the LTM9012 may also be decreased from
the nominal value by adding resistance in series with
the signal inputs. The internal op amps are fed by 49.9Ω
series resistors and employ 511Ω feedback resistors. The
voltage gain of the stage is set by the ratio of the feedback
resistance to the total series resistance. Unity gain, for
example, can be realized by adding a 464Ω resistor in
series with each input.
Reference
The LTM9012 has an internal 1.25V voltage reference. For
a 2V input range using the internal reference with a unity
gain internal amplifier configuration, connect SENSE to V_DD.
For a 1V input range using the internal reference, connect
SENSE to ground. For a 2V input range with an external
reference, apply a 1.25V reference voltage to SENSE.
The input range can be adjusted by applying a voltage to
SENSE that is between 0.625V and 1.30V. The input range
will then be 1.6 • V_SENSE.
The reference is shared by all four ADC channels, so it is
not possible to independently adjust the input range of
individual channels.
9012f
14