Datasheet

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SBAS274H − MARCH 2003 − REVISED MAY 2007
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18
INPUT CIRCUITRY
The ADS1605/6 uses switched-capacitor circuitry to
measure the input voltage. Internal capacitors are
charged by the inputs and then discharged internally
with this cycle repeating at the frequency of CLK.
Figure 8 shows a conceptual diagram of these circuits.
Switches S2 represent the net effect of the modulator
circuitry in discharging the sampling capacitors; the ac-
tual implementation is different. The timing for switches
S1 and S2 is shown in Figure 9.
S
1
S
2
10pF
AINP
ADS1605
ADS1606
8pF
VMID
S
1
S
2
10pF
AINN
8pF
VMID
AGND
Figure 8. Conceptual Diagram of Internal
Circuitry Connected to the Analog Inputs
On
S1
S2
Off
On
Off
t
SAMPLE
=1/f
CLK
Figure 9. Timing for the Switches in Figure 2
DRIVING THE INPUTS
The external circuits driving the ADS1605/6 inputs must
be able to handle the load presented by the switching
capacitors within the ADS1605/6. The input switches
S1 in Figure 8 are closed approximately one half of the
sampling period, t
sample
, allowing only ≈12ns for the in-
ternal capacitors to be charged by the inputs, when f
CLK
= 40MHz.
Figure 10 and Figure 11 show the recommended cir-
cuits when using single-ended or differential op amps,
respectively. The analog inputs must be driven differen-
tially to achieve optimum performance. The external ca-
pacitors, 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 in-
puts and use good capacitor bypass techniques on their
supplies; usually a smaller high-quality ceramic capaci-
tor in parallel with a larger capacitor. Keep the resistanc-
es used in the driver circuits low—thermal noise in the
driver circuits degrades the overall noise performance.
When the signal can be ac-coupled to the ADS1605/6
inputs, a simple RC filter can set the input common
mode voltage. The ADS1605/6 is a high-speed, high-
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 ADS1605/6.
392
Ω
OPA2822
ADS1605
ADS1606
AGND
OPA2822
40pF
V
CM
(1)
V
CM
(1)
V
CM
(1)
100pF
AINP
AINN
100pF
(3)
392
Ω
40pF
100pF
(1) Recommended V
CM
=2.0V.
(2) Optional ac−coupling circuit provides common−mode input voltage.
(3) Increase to 390pF when f
IN
≤
100kHz for improved SNR and THD.
(2)
(2)
(2)
(2)
−
V
IN
2
V
IN
2
392
Ω
392
Ω
392
Ω
0.01
µ
F
0.01
µ
F
392
Ω
1
µ
F392
Ω 1k
Ω
1k
Ω
1
µ
F392
Ω
49.9
Ω
49.9
Ω
Figure 10. Recommended Driver Circuit Using the
OPA2822
ADS1605
THS4503
ADS1606
22pF
−
V
IN
+V
IN
100pF
100pF
AINP
AINN
100pF
24.9
Ω
392
Ω
392
Ω
24.9
Ω
392
Ω
392
Ω
22pF
V
CM
Figure 11. Recommended Driver Circuits Using
the THS4503 Differential Amplifier