LMH6629_1011 Datasheet, PDF (24/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

Figure 22 shows a ground referenced bipolar input (symmet-
rical swing around 0V) SE to differential converter used to
drive a high resolution ADC. The combination of LMH6629’s
low noise and the converter architecture reduces the impact
on the ADC noise.
FIGURE 22. Low-Noise Single-Ended (SE) to Differential Converter
30068055
In this circuit, the required gain dictates the resistor ratio “K”.
With “K” and the driver output CM voltage (V_{O_CM}) known,
V_SET can be established. Reasonable values for R_f and R_g
can be set to complete the design.
In terms of output swing, with the LMH6629 output swing ca-
pability which requires ~0.85V of headroom from either rail,
the maximum total output swing into the ADC is limited to 6.6
V_PP (=(5 – 2 x 0.85V) x 2); that is true with V_{O_CM} set to mid-
rail between V⁺ and V^-. It should also be noted that the
LMH6629’s input CMVR range includes the lower rail (V^-) and
that is the reason there is great flexibility in setting V_{o_CM} by
controlling V_SET. Another feature is that A1 and A2 inputs act
like “virtual grounds” and thus do not see any signal swing.
Note that due to the converter’s biasing, the source, V_IN,
needs to sink a current equal to V_SET / R_IN.
The converter example shown in Figure 22 operates with a
noise gain of 6 (=1+ K / 2) and thus requires that the COMP
pin to be tied low (LLP-8 package only). The 1^st order ap-
proximated small signal bandwidth will be 280 MHz (=1.7
GHz / 6V/V) which is computed using 1.7GHz as the GBWP
with COMP pin LO .
From a noise point of view, concentrating only on the domi-
nant noise sources involved, here is the expression for the
expected differential noise density at the input of the ADC:
Equation 9: Converter Noise Expression
(9)
e_n is the LMH6629 input noise voltage and e_{Rin_thermal} is the
thermal noise of R_IN. The “2³â€ and the “2²â€ multipliers account
for the different instances of each noise source (2 for e_n, and
1 for e_{Rin_thermal}.
Equation 9 evaluated for the circuit example of Figure 22 is
shown below:
Equation 10: Converter Noise Expression
Evaluated
(10)
Because of the LMH6629’s low input noise voltage (e_n), noise
is dominated by the thermal noise of R_IN. It is evident that the
input resistor, R_IN, can be reduced to lower the noise with
lower input impedance as the trade-off.
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