THS770012 Datasheet, PDF (22/35 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC Driver Amplifier

THS770012
SLOS669C – FEBRUARY 2010 – REVISED JANUARY 2012
www.ti.com
For common-mode analysis, assume that V_AMP± = V_OCM and V_ADC± = V_ADC (the specification for the ADC input
common-mode voltage). V_REF 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 R_O is chosen to be a
reasonable value, such as 49.9Ω. With these known values, R_P can be found by using Equation 1:
R_P = R_O
V_ADC - V_REF
V_AMP - V_ADC
(1)
The insertion of this resistor network also attenuates the amplifier output signal. The gain (or loss) can be
calculated by Equation 2:
GAIN =
2R_P || Z_IN
2R_O + 2R_P || Z_IN
(2)
Using the gain and knowing the full-scale input of the ADC, V_{ADC FS}, the required amplitude to drive the ADC with
the network can be calculated using Equation 3:
V_{AMP PP}
=
V_{ADC FS}
GAIN
(3)
Using the ADC examples given previously, Table 3 shows sample calculations of the value of R_P and V_{AMP 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.
ADC
ADS5485
ADS6149
ADS4149
ADS4149
V_OCM
(V_DC)
2.5
2.5
2.5
0 ⁽¹⁾
V_ADC
(V_DC)
3.1
1.5
0.95
0.95
Table 3. Example R_P for Various ADCs
V_REF
R_INT
R_O
(V_DC)
(Ω)
(Ω)
5
1k
50
R_P
(Ω)
158.3
GAIN
(V/V)
0.73
GAIN
(dB)
–2.71
0
NA
50
75.0
0.60
–4.44
0
NA
50
30.6
0.38
–8.40
2.5
NA
50
81.6
0.62
–4.15
V_{ADC FS}
(V_PP)
2
2
2
2
V_{AMP PP}
FS (V_PP)
4.10
3.33
5.26
3.23
V_{AMP PP}
–1dBFS (V_PP)
3.65
2.97
4.69
2.88
(1) THS770012 with ±2.5V supply.
The calculated values for the ADS5485 give the lowest attenuation, and because of the high V_FS, it requires
3.65V_PP 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 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 R_INT 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 5.26V_PP 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 2.88V_PP is required for the –1dBFS input to the ADS4149. See the Operation with Split
Supply ±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 R_INT in Equation 2.
As with any design, testing is recommended to validate whether it meets the specific design goals.
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