MAX1205 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – +5v sINGLE-sUPPLY, 1mSPS, 14-bIT sELF-cALIBRATING adc

+5V Single-Supply, 1Msps, 14-Bit
Self-Calibrating ADC
external amplifiers to compensate for any resistive drop
on these lines, internal or external to the chip. Ensure a
correct reference voltage by using proper Kelvin con-
nections at the sense pins.
Common-Mode Voltage
The switched capacitor input circuit at the analog input
allows signals between AGND and the analog power
supply. Since the common-mode voltage has a strong
influence on the performance of the ADC, the best
results are obtained by choosing V_CM to be at half the
difference between the reference voltages V_RFP and
V_RFN. Achieve this by using a resistive divider between
the two reference potentials. Figure 4 shows a typical
driving circuit for good dynamic performance.
Analog Signal Conditioning
For single-ended inputs the negative analog input pin
(INN) is connected to the common-mode voltage pin
(CM), and the positive analog input pin (INP) is con-
nected to the input.
To take full advantage of the ADC’s superior AC perfor-
mance up to the Nyquist frequency, drive the chip with
differential signals. In communication systems, the sig-
nals may inherently be available in differential mode.
Medical and/or other applications may only provide sin-
gle-ended inputs. In this case, convert the single-
ended signals into differential ones by using the circuit
recommended in Figure 5. Use low-noise, wideband
amplifiers such as the MAX4108 to maintain the signal
purity over the full-power bandwidth of the MAX1205
input.
Lowpass or bandpass signals may be required to
improve the signal-to-noise-and-distortion ratio of the
incoming signal. For low-frequency signals (<100kHz),
active filters may be used. For higher frequencies, pas-
sive filters are more convenient.
Single-Ended to Differential
Conversion Using Transformers
An alternative single-ended to differential-ended con-
version method is a balun transformer such as the
CTX03-13675 from Coiltronics. An important benefit of
these transformers is their ability to level-shift single-
ended signals referred to ground on the primary side to
optimum common-mode voltages on the secondary
side. At frequencies below 20kHz, the transformer core
begins to saturate, causing odd-order harmonics.
Clock Source Requirements
Pipelined ADCs typically need a 50% duty cycle clock.
To avoid this constraint, the MAX1205 provides a
RFP = 4.096V
5k
MAX410
5k
RFN = 0V
MAX410
MAX410
V+
CHIP BOUNDARY
RFPF
IN
INP
MAX4108
RFPS
CM
V-
RFNF
RFNS
CM
V+
INN
MAX4108
V-
CM
Figure 4. Drive Circuit for the Reference Pins and the Common-
Mode Pin
Figure 5. A simple circuit generates differential signals from a
single-ended input referred to analog ground. The common-
mode voltage at INP and INN is the same as CM.
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