參數(shù)資料
型號(hào)： THS770012IRGET
廠商： TEXAS INSTRUMENTS INC
元件分類： 模擬信號(hào)調(diào)理
英文描述： SPECIALTY ANALOG CIRCUIT, PQCC24
封裝： GREEN, PLASTIC, VQFN-24
文件頁(yè)數(shù)： 15/35頁(yè)
文件大小： 1305K
代理商： THS770012IRGET
V
ADC
REF
-
V
AMP
ADC
-
R
P
O
=R
GAIN=
2R ||Z
P
IN
2R +2R ||Z
O
P
IN
V
ADC FS
GAIN
V
AMP PP
=
SLOS669B
– FEBRUARY 2010 – REVISED MAY 2011
For common-mode analysis, assume that VAMP± = VOCM and VADC± = VADC (the specification for the ADC input
common-mode voltage). VREF is chosen to be a voltage within the system (such as the ADC or amplifier analog
supply) or ground, depending on whether the voltage must be pulled up or down, and RO is chosen to be a
reasonable value, such as 49.9
Ω. With these known values, RP can be found by using Equation 1:
(1)
The insertion of this resistor network also attenuates the amplifier output signal. The gain (or loss) can be
calculated by Equation 2:
(2)
Using the gain and knowing the full-scale input of the ADC, VADC FS, the required amplitude to drive the ADC with
the network can be calculated using Equation 3:
(3)
Using the ADC examples given previously, Table 3 shows sample calculations of the value of RP and VAMP FS for
full-scale drive, and then for
–1dB (often times, the ADC drive is backed off from full-scale in applications, so
lower amplitudes may be acceptable). All voltages are in volts, resistors in
Ω (the nearest standard value should
be used), and gain as noted. Table 3 does not include the ADS5424 because no level shift is required with this
device.
Table 3. Example RP for Various ADCs
VOCM
VADC
VREF
RINT
RO
RP
GAIN
VADC FS
VAMP PP
ADC
(VDC)
(
Ω)
(
Ω)
(
Ω)
(V/V)
(dB)
(VPP)
FS (VPP)
–1dBFS (VPP)
ADS5485
2.5
3.1
5
1k
50
158.3
0.73
–2.71
2
4.10
3.65
ADS5493
2.5
3.15
5
1k
50
142.3
0.71
–2.93
2.5
3.50
3.12
ADS6149
2.5
1.5
0
NA
50
75.0
0.60
–4.44
2
3.33
2.97
ADS4149
2.5
0.95
0
NA
50
30.6
0.38
–8.40
2
5.26
4.69
ADS4149
0(1)
0.95
2.5
NA
50
81.6
0.62
–4.15
2
3.23
2.88
(1)
THS770012 with
±2.5V supply.
The calculated values for the ADS5485 give the lowest attenuation, and because of the high VFS, it requires
3.65VPP from the amplifier to drive to –1dBFS. Performance of the THS770012 is still very good up to 130MHz at
this level, but the designer may want to further back off from full-scale for best performance and consider trading
reduced SNR performance for better SFDR performance.
The calculated values for the ADS5493 have lower attenuation as a result of reduced VFS, and requires 3.12VPP
from the amplifier to drive to
–1dBFS. Performance of the THS770012 is excellent at this level up to 130MHz.
The values calculated for the ADS6149 show reasonable design targets and should work with good performance.
Note the ADS6149 does not have buffered inputs, and the inputs have equivalent resistive impedance that varies
with sampling frequency. In order to account for the increased loss, half of this resistance should be used for the
value of RINT in Equation 2.
The values calculated for the low input common-mode of the ADS4149 result in large attenuation of the amplifier
signal leading to 5.26VPP being required for full-scale ADC drive. This amplitude is greater than the maximum
capability of the device. With a single +5V supply, the THS770012 is not suitable to drive this ADC in dc-coupled
applications unless the ADC input is backed off towards
–6dBFS. Another option is to operate the THS770012
with a split
±2.5V supply, and is shown in the last row of Table 3. For this situation, if the +2.5V is used as the
pull-up voltage, only 2.88VPP is required for the –1dBFS input to the ADS4149. See the Operation with Split
±2.5V section for more detail. Note that the ADS4149 does not have buffered inputs and the inputs have
equivalent resistive impedance that varies with sampling frequency. In order to account for the increased loss,
half of this resistance should be used for the value of RINT in Equation 2.
As with any design, testing is recommended to validate whether it meets the specific design goals.
22
Copyright
2010–2011, Texas Instruments Incorporated
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