AD9240EB データシートの表示（PDF） - Analog Devices
部品番号
コンポーネント説明
メーカー
AD9240EB Datasheet PDF : 24 Pages
| |||
AD9240
The addition of a differential input structure gives the user an
additional level of flexibility that is not possible with traditional
flash converters. The input stage allows the user to easily con-
figure the inputs for either single-ended operation or differential
operation. The A/D’s input structure allows the dc offset of the
input signal to be varied independently of the input span of the
converter. Specifically, the input to the A/D core is the differ-
ence of the voltages applied at the VINA and VINB input pins.
Therefore, the equation,
VCORE = VINA – VINB
(1)
defines the output of the differential input stage and provides
the input to the A/D core.
The voltage, VCORE, must satisfy the condition,
–VREF ≤ VCORE ≤ VREF
(2)
where VREF is the voltage at the VREF pin.
While an infinite combination of VINA and VINB inputs exist
that satisfy Equation 2, there is an additional limitation placed
on the inputs by the power supply voltages of the AD9240. The
power supplies bound the valid operating range for VINA and
VINB. The condition,
AVSS – 0.3 V < VINA < AVDD + 0.3 V
(3)
AVSS – 0.3 V < VINB < AVDD + 0.3 V
where AVSS is nominally 0 V and AVDD is nominally +5 V,
defines this requirement. Thus, the range of valid inputs for
VINA and VINB is any combination that satisfies both Equa-
tions 2 and 3.
For additional information showing the relationship between
VINA, VINB, VREF and the digital output of the AD9240, see
Table IV.
Refer to Table I and Table II for a summary of the various
analog input and reference configurations.
ANALOG INPUT OPERATION
Figure 24 shows the equivalent analog input of the AD9240
which consists of a differential sample-and-hold amplifier (SHA).
The differential input structure of the SHA is highly flexible,
allowing the devices to be easily configured for either a differen-
tial or single-ended input. The dc offset, or common-mode
voltage, of the input(s) can be set to accommodate either single-
supply or dual supply systems. Note also that the analog inputs,
VINA and VINB, are interchangeable with the exception that
reversing the inputs to the VINA and VINB pins results in a
polarity inversion.
VINA
VINB
CPIN+
CPAR
QS1
QS1
CPIN–
CPAR
CS
QH1 CS
CH
QS2
QS2
CH
The input SHA of the AD9240 is optimized to meet the perfor-
mance requirements for some of the most demanding commu-
nication, imaging, and data acquisition applications while
maintaining low power dissipation. Figure 25 is a graph of the
full-power bandwidth of the AD9240, typically 60 MHz. Note
that the small signal bandwidth is the same as the full-power
bandwidth. The settling time response to a full-scale stepped
input is shown in Figure 26 and is typically less than 40 ns to
0.0025%. The low input referred noise of 0.36 LSB’s rms is
displayed via a grounded histogram and is shown in Figure 13.
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
1
10
100
FREQUENCY – MHz
Figure 25. Full-Power Bandwidth
16000
12000
8000
4000
0
0
10 20 30 40 50 60 70 80
SETTLING TIME – ns
Figure 26. Settling Time
The SHA’s optimum distortion performance for a differential or
single-ended input is achieved under the following two condi-
tions: (1) the common-mode voltage is centered around mid-
supply (i.e., AVDD/2 or approximately 2.5 V) and (2) the input
signal voltage span of the SHA is set at its lowest (i.e., 2 V input
span). This is due to the sampling switches, QS1, being CMOS
switches whose RON resistance is very low but has some signal
dependency which causes frequency dependent ac distortion
while the SHA is in the track mode. The RON resistance of a
CMOS switch is typically lowest at its midsupply but increases
symmetrically as the input signal approaches either AVDD or
AVSS. A lower input signal voltage span centered at midsupply
reduces the degree of RON modulation.
Figure 24. Simplified Input Circuit
REV. A
–9–
Share Link: 