LMH6629_1011 Datasheet, PDF (23/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

30068036
FIGURE 19. Low Pass Sallen-Key Active Filter Topology
LOW-NOISE MAGNETIC MEDIA EQUALIZER
Figure 20 shows a high-performance low-noise equalizer for
such applications as magnetic tape channels using the
LMH6629. The circuit combines an integrator (used to limit
noise) with a bandpass filter (used to boost the response cen-
tered at a frequency or over a band of interest) to produce the
low-noise equalization. The circuit’s simulated frequency re-
sponse is illustrated in Figure 21.
In this circuit, the bandpass filter center frequency is set by
30068031
FIGURE 20. Low-Noise Magnetic Media Equalizer
For higher selectivity, use high C values; for wider bandwidth,
use high L values, while keeping the product of L and C values
the same to keep f_c intact. The integrator’s -3dB roll-off is set
by
If
the integrator and the bandpass filter frequency interaction is
minimized so that the operating frequencies of each can be
set independently. Lowering the value of R2 increases the
bandpass gain (boost) without affecting the integrator fre-
quencies. With the LMH6629’s wide Gain Bandwidth (4GHz),
the center frequency could be adjusted higher without worries
about loop gain limitation. This increases flexibility in tuning
the circuit.
30068032
FIGURE 21. Equalizer Frequency Response
LOW-NOISE SINGLE ENDED TO DIFFERENTIAL
CONVERTER / DRIVER
Many high-resolution data converters (ADC’s) require a dif-
ferential input driver. In order to preserve the ADC’s dynamic
range, the analog input driver must have a noise floor which
is lower than the ADC’s noise floor. For an ADC with N bits,
the quantization Signal-to-noise ratio (SNR) is 6.02* N + 1.76
in dB. For example, a 12-bit ADC has a SNR of 74 dB (= 5000
V/V). Assuming a full-scale differential input of 2Vpp (0.707
V_RMS), the quantization noise referred to the ADC’s input
is ~140 μV_RMS (= 0.707 V_RMS / 5000 V/V) over the band-
width “visible” to the ADC. Assuming an ADC input bandwidth
of 20 MHz, this translates to just 25 nV/RtHz (= 141µV_RMS /
SQRT(20 MHz * π/2)) noise density at the output of the driver.
Using an amplifier to form the single-ended (SE) to Differential
converter / driver for such an application is challenging, es-
pecially when there is some gain required. In addition, the
input driver’s linearity (harmonic distortion) must also be high
enough such that the spurs that get through to the ADC input
are below the ADC’s LSB threshold or -73 dBc (= 20*log (1/
2¹²)) or lower in this case. Therefore, it is essential to use a
low-noise / low-distortion device to drive a high resolution
ADC in order to minimize the impact on the quantization noise
and to make sure that the driver’s distortion does not domi-
nate the acquired data.
23
www.national.com