LMH6401_16 Datasheet, PDF (34/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
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A second approach is shown in Figure 68 when dc-coupling on a single supply, where a resistor network can be
used to perform the common-mode level shift. This resistor network consists of the amplifier series output
resistors and pullup or pulldown resistors to a reference voltage. This resistor network introduces signal
attenuation that may prevent the use of the full-scale input range of the ADC. ADCs with an input common-mode
closer to the typical 2.5-V output common-mode of the LMH6401 are easier to dc-couple, and require little or no
level shifting.
LMH6401
+5 V
V_REF = GND
V_AMP+
R_P
R_O
V_ADC+
10 Ÿ
10 Ÿ
V_AMP-
R_O
R_P
R_IN CA_INDS4A1DBC49
V_ADC-
GND
V_REF = GND
Figure 68. Resistor Network to DC Level-Shift Common-Mode Voltage using V_REF as GND
For common-mode analysis of the circuit in Figure 68, assume that V_AMP± = V_CM and V_ADC± = V_CM (the
specification for the ADC input common-mode voltage). Note that the V_AMP± common-mode voltage is set before
the two internal 10-Ω resistors, making these resistors necessary to include in the common-mode level-shift
resistor calculation. V_REF is chosen to be a voltage within the system higher than V_CM (such as the ADC or
amplifier analog supply) or ground, depending on whether the voltage must be pulled up or down, respectively;
R_O is chosen to be a reasonable value, such as 24.9 Ω. With these known values, R_P can be found by using
Equation 6:
R_P= (10 + R_O) × (V_ADC – V_REF) / (V_AMP – V_ADC)
(6)
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 7:
Gain = (2R_P || Z_IN) / (20 + 2R_O + 2R_P || Z_IN)
(7)
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 ADS54J60 has a 0.6-kΩ resistor
tying each input to the ADC V_CM; therefore, the effective differential R_IN is 1.2 kΩ.
The introduction of the R_P resistors also modifies the effective load that must be driven by the amplifier.
Equation 8 shows the effective load created when using the R_P resistors.
R_L = 20 + 2R_O + 2R_P || Z_IN
(8)
The R_P resistors function in parallel to the ADC input such that the effective load (output current) at the amplifier
output is increased. Higher current loads limit the LMH6401 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 9:
V_{AMP PP}
=
V_{ADC FS}
GAIN
(9)
As with any design, testing is recommended to validate whether the specific design goals are met.
34
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