THS770006 Datasheet, PDF (23/39 Pages) National Semiconductor (TI) – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
www.ti.com
SBOS520B – JULY 2010 – REVISED JANUARY 2012
Shifting the common-mode voltage with the resistor network comes at the expense of signal attenuation.
Modeling the ADC input as the parallel combination of a resistance (R_IN) and capacitance (C_IN) using values
taken from the ADC data sheet, the approximate differential input impedance (Z_IN) for the ADC can be calculated
at the signal frequency. The effect of C_IN on the overall calculation of gain is typically minimal and can be ignored
for simplicity (that is, Z_IN = R_IN). The ADC input impedance creates a divider with the resistor network; the gain
(attenuation) for this divider can be calculated by Equation 5:
GAIN =
2R_P || Z_IN
2R_O + 2R_P || Z_IN
(5)
With ADCs that have internal resistors that bias the ADC input to the ADC input common-mode voltage, the
effective R_IN is equal to twice the value of the bias resistor. For example, the ADS5485 has a 1kΩ resistor tying
each input to the ADC V_CM; therefore, the effective differential R_IN is 2kΩ.
The introduction of the R_P resistors also modifies the effective load seen by the amplifier. Equation 6 shows the
effective load seen by the amplifier:
R_L = 2R_O + 2R_P || Z_IN
(6)
The R_P resistors act in parallel to the ADC input such that the effective load (output current) seen by the amplifier
is increased. Higher current loads limit the THS770006 differential output swing.
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 7:
V_{AMP PP}
=
V_{ADC FS}
GAIN
(7)
Using the ADC examples given previously, Table 2 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 voltage values are in volts, resistor values in ohms (the nearest
standard value should be used), and gain values are as noted. Table 2 does not include the ADS5424 because
no level shift is required with this device.
Table 2. Example R_P for Various ADCs
ADC
ADS5485
ADS6149
ADS4149
V_AMP
(V_DC)
2.5
2.5
2.5
0 ⁽²⁾
0 ⁽²⁾
V_ADC
(V_DC)
3.1
1.5
0.95
0.95
0.95
V_REF
ADC R_IN
R_O
(V_DC)
(Ω)
(Ω)
5
2k
50
0
6k ⁽¹⁾
50
0
5k ⁽¹⁾
50
2.5
5k ⁽¹⁾
50
5
5k ⁽¹⁾
50
(1) At 70MHz.
(2) THS770006 with ±2.5V supply.
R_P
(Ω)
158.3
75.0
30.6
81.6
213.2
GAIN
(V/V)
0.73
0.59
0.38
0.61
0.79
GAIN
(dB)
–2.72
–4.56
–8.49
–4.31
–2.05
V_{ADC FS}
(V_PP)
3
2
2
2
2
V_{AMP PP}
FOR
0dBFS(V
PP⁾
4.10
3.38
5.31
3.28
2.53
V_{AMP PP}
FOR –1d
BFS
(V_PP)
3.66
3.01
4.74
2.93
2.26
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