ADS5510
www.ti.com
SLAS499–JANUARY 2007
This differential input topology produces a high level of ac-performance for high sampling rates. It also results in
a very high usable input bandwidth, especially important for high intermediate-frequency (IF) or undersampling
applications. The ADS5510 requires each of the analog inputs (INP, INM) to be externally biased around the
common-mode level of the internal circuitry (CM, pin 17). For a full-scale differential input, each of the differential
lines of the input signal (pins 19 and 20) swings symmetrically between CM + 0.575 V and CM – 0.575 V. This
means that each input is driven with a signal of up to CM ± 0.575 V, so that each input has a maximum
differential signal of 1.15 VPP for a total differential input signal swing of 2.3 VPP. The maximum swing is
determined by the two reference voltages, the top reference (REFP, pin 29), and the bottom reference (REFM,
pin 30).
The ADS5510 obtains optimum performance when the analog inputs are driven differentially. The circuit shown
in Figure 25 illustrates one possible configuration using an RF transformer.
R0
Z0
Ω
50
Ω
25
Ω
50
INP
1:1
R
50
AC Signal
Source
ADS5510
Ω
25
Ω
INM
CM
ADT1−1WT
Ω
10
µ
0.1 F
1nF
Figure 25. Transformer Input to Convert Single-Ended Signal to Differential Signal
The single-ended signal is fed to the primary winding of an RF transformer. Placing a 25-Ω resistor in series with
INP and INM is recommended to dampen ringing due to ADC kickback.
Since the input signal must be biased around the common-mode voltage of the internal circuitry, the
common-mode voltage (VCM) from the ADS5510 is connected to the center-tap of the secondary winding.
To ensure a steady low-noise VCM reference, best performance is attained when the CM output (pin 17) is
filtered to ground with a 10-Ω series resistor and parallel 0.1-µF and 0.001-µF low-inductance capacitors, as
illustrated in Figure 24.
Output VCM (pin 17) is designed to directly drive the ADC input. When providing a custom CM level, be aware
that the input structure of the ADC sinks a common-mode current in the order of 600 µA (300 µA per input).
Equation 1 describes the dependency of the common-mode current and the sampling frequency:
600mA fS (in MSPS)
125 MSPS
(1)
Where:
fS > 2 MSPS.
This equation helps to design the output capability and impedance of the driving circuit accordingly.
When it is necessary to buffer or apply a gain to the incoming analog signal, it is possible to combine
single-ended operational amplifiers with an RF transformer, or to use a differential input/output amplifier without
a transformer, to drive the input of the ADS5510. Texas Instruments offers a wide selection of single-ended
operational amplifiers (including the THS3201, THS3202, OPA695, and OPA847) that can be selected
depending on the application. An RF gain block amplifier, such as Texas Instruments THS9001, can also be
used with an RF transformer for high input frequency applications. The THS4503 is a recommended differential
input/output amplifier. Table 4 lists the recommended amplifiers.
19
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