ADS1610
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SBAS344C–AUGUST 2005–REVISED OCTOBER 2006
driver circuits low-thermal noise in the driver circuits
degrades the overall noise performance. When the
signal can be AC-coupled to the ADS1610 inputs, a
simple RC filter can set the input common mode
ADS1610
S1
AINP
voltage. The ADS1610 is
a high-speed, high-
S2
performance ADC. Special care must be taken when
selecting the test equipment and setup used with this
device. Pay particular attention to the signal sources
to ensure they do not limit performance when
measuring the ADS1610.
10pF
8pF
VMID
S1
AINN
S2
10pF
8pF
10pF
ADS1610
VMID
Ω
787
AGND
Ω
Ω
Ω
374
12.5
12.5
VIN
AINP
AINN
Figure 25. Conceptual Diagram of Internal
Circuitry Connected to the Analog Inputs
100pF
100pF
Ω
56.2
VCM
THS4503
Ω
374
−VIN
100pF
Ω
787
Ω
56.2
tSAMPLE = 1/fCLK
On
Off
S1
S2
10 pF
On
Off
Figure 27. Recommended Single-Ended to
Differential Conversion Circuit Using the
THS4503 Differential Amplifier
Figure 26. Timing for the Switches in Figure 25
DRIVING THE INPUTS
Ω
392
The external circuits driving the ADS1610 inputs
must be able to handle the load presented by the
switching capacitors within the ADS1610. The input
switches S1 in Figure 25 are closed approximately
one half of the sampling period, tSAMPLE, allowing
only ~8 ns for the internal capacitors to be charged
by the inputs, when fCLK = 60MHz.
20 pF
Ω
Ω
392
392
V
IN
−
2
µ
0.01
F
Ω
12.5
AINP
OPA2822
(2)
(1)
V
CM
100pF
Ω
1k
Ω
µ
1 F
392
(2)
Figure 27 and Figure 28 show the recommended
circuits when using single-ended or differential op
amps, respectively. The analog inputs must be
driven differentially to achieve optimum performance.
If only a single-ended input signal is available, the
configuration in Figure 27 can be used by shorting
–VIN to ground.
Ω
392
ADS1610
(1)
(3)
100pF
V
CM
(2)
20 pF
Ω
392
V
Ω
1k
IN
2
µ
F
0.01
Ω
12.5
AINN
OPA2822
Ω
392
(2)
(1)
V
CM
100pF
Ω
µ
F
392
1
This configuration would implement the single-ended
to differential conversion.
A GND
The external capacitors, between the inputs and from
each input to AGND, improve linearity and should be
placed as close to the pins as possible. Place the
drivers close to the inputs and use good capacitor
bypass techniques on their supplies; usually a
smaller high-quality ceramic capacitor in parallel with
a larger capacitor. Keep the resistances used in the
(1) Recommended VCM = 2.5V.
(2) Optional ac−coupling circuit provides common−mode input voltage.
≤
(3) Increase to 390pF when fIN 100kHz for improved SNR and THD.
Figure 28. Recommended Driver Circuit Using
the OPA2822
13
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