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參數資料
| 型號： | EVAL-AD7949EDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數： | 13/32頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL AD7949 |
| 標準包裝： | 1 |
| 系列： | PulSAR® |
| ADC 的數量： | 1 |
| 位數： | 14 |
| 采樣率（每秒）： | 250k |
| 數據接口： | 串行 |
| 輸入范圍： | ±VREF |
| 在以下條件下的電源（標準）： | 10.8mW @ 250kSPS，5V |
| 工作溫度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7949 |
| 已供物品： | 板 |
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AD7949
Data Sheet
Rev. D | Page 20 of 32
External Reference and Internal Buffer
For improved drift performance, an external reference can be
used with the internal buffer, as shown in Figure 31. The
external source is connected to REFIN, the input to the on-chip
unity gain buffer, and the output is produced on the REF pin.
An external reference can be used with the internal buffer with
or without the temperature sensor enabled. Refer to Table 9 for
register details. With the buffer enabled, the gain is unity and is
limited to an input/output of VDD = 0.2 V; however, the
maximum voltage allowable must be ≤(VDD 0.5 V).
The internal reference buffer is useful in multiconverter appli-
cations because a buffer is typically required in these applications.
In addition, a low power reference can be used because the
internal buffer provides the necessary performance to drive the
SAR architecture of the AD7949.
REF
GND
TEMP
AD7949
10F
100nF
REFIN
REF SOURCE
≤ (VDD – 0.5V)
0
73
51
-0
31
Figure 31. External Reference Using Internal Buffer
External Reference
In any of the six voltage reference schemes, an external reference
can be connected directly on the REF pin as shown in Figure 32
because the output impedance of REF is >5 kΩ. To reduce power
consumption, the reference and buffer should be powered down.
When using only the external reference (and optional reference
buffer as shown in Figure 35), the internal buffer is disabled.
Refer to Table 9 for register details. For improved drift perfor-
mance, an external reference such as the ADR43x or ADR44x is
recommended.
REF
GND
TEMP
10F
REFIN
REF SOURCE
0.5V < REF < (VDD + 0.3V)
NO CONNECTION
REQUIRED
AD7949
07
35
1-
03
2
Figure 32. External Reference
Note that the best SNR is achieved with a 5 V external reference
as the internal reference is limited to 4.096 V. The SNR
degradation is as follows:
5
096
.
4
log
20
LOSS
SNR
Reference Decoupling
Whether using an internal or external reference, the AD7949
voltage reference output/input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins. This decoupling depends on the choice of the voltage
reference but usually consists of a low ESR capacitor connected
to REF and GND with minimum parasitic inductance. A 10 μF
(X5R, 1206 size) ceramic chip capacitor is appropriate when
using the internal reference, the ADR43x/ADR44x external
reference, or a low impedance buffer such as the AD8031 or
The placement of the reference decoupling capacitor is also impor-
tant to the performance of the AD7949, as explained in the
Layout section. Mount the decoupling capacitor on the same side
as the ADC at the REF pin with a thick PCB trace. The GND
should also be connected to the reference decoupling capacitor
with the shortest distance and to the analog ground plane with
several vias.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with minimal impact on performance,
especially on DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
For applications that use multiple AD7949 devices or other
PulSAR devices, it is more effective to use the internal reference
buffer to buffer the external reference voltage, thus reducing
SAR conversion crosstalk.
The voltage reference temperature coefficient (TC) directly
impacts full scale; therefore, in applications where full-scale
accuracy matters, care must be taken with the TC. For instance,
a ±10 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
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