LMH6611_14 Datasheet, PDF (24/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

LMH6611, LMH6612
SNOSB00K – NOVEMBER 2007 – REVISED OCTOBER 2013
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Important Specifications of Op Amp and ADC
When interfacing an ADC with an op amp it is imperative to understand the specifications that are important to
get the expected performance results. Modern ADC AC specifications such as THD, SNR, settling time and
SFDR are critical for filtering, test and measurement, video and reconstruction applications. The high
performance op amp’s settling time, THD, and noise performance must be better than that of the ADC it is driving
to maintain the proper system accuracy with minimal or no error.
Some system applications require low THD, low SFDR and wide dynamic range (SNR), whereas some system
applications require high SNR and they may sacrifice THD and SFDR to focus on the noise performance.
Noise is a very important specification for both the op amp and the ADC. There are three main sources of noise
that contribute to the overall performance of the ADC: Quantization noise, noise generated by the ADC itself
(particularly at higher frequencies) and the noise generated by the application circuit. The impedance of the input
source affects the noise performance of the op amp. Theoretically, an ADC’s signal to noise ratio (SNR) can be
found from the equation:
SNR (in dB) = 6.02*N+1.72
(1)
where N is the resolution of the ADC. For example, according to this equation a 12-bit ADC has an SNR of 74
dB. However, the practical SNR number would be about 72 dB. In order to achieve better SNR, the ADC driver
noise should be as small as possible. The LMH6611/LMH6612 have the low voltage noise of only 10 nV/√Hz.
The combined settling time of the op amp and the ADC must be within 1 LSB. The 0.01% settling time of the
LMH6611/LMH6612 is 100 ns.
The ADC driver’s THD should be inherently lower than that of the ADC. The LMH6611/LMH6612 have an SFDR
of 96 dBc at 2 V_PP output and 1 MHz input frequency.
Signal to Noise and Distortion (SINAD) is a parameter which is the combination of the SNR and THD
specifications. SINAD is defined as the RMS value of the output signal to the RMS value of all of the other
spectral components below half the clock frequency, including harmonics but excluding DC. It can be calculated
from SNR and THD according to the equation:
-SNR
THD
SINAD = 20 * LOG 10 ¹⁰ + 10¹⁰
(2)
Because SINAD compares all undesired frequency components with the input frequency, it is an overall measure
of an ADC’s dynamic performance. The following sections will discuss the three different ADC driver
architectures in detail.
SINGLE TO SINGLE ADC DRIVER
This architecture has a single-ended input source connected to the input of the op amp and the single-ended
output of the op amp is then fed to the single-ended input of the ADC. The low noise of only 10 nV/√Hz and a
wide bandwidth of 345 MHz make the LMH6611 an excellent choice for driving the 12-bit ADC121S101 500
KSPS to 1 MSPS ADC, which has a successive approximation architecture with internal sample and hold
circuits. Figure 67 shows the schematic of the LMH6611 in a 2nd order multiple-feedback with gain of −1
(inverting) configuration, driving an ADC121S101. The inverting configuration is preferred over the non-inverting
configuration, as it offers more linear output response. Table 1 shows the performance data of the LMH6611
combined with the ADC121S101. The ADC driver’s cutoff frequency of 500 kHz is found from the equation:
¶´0
=
1
2S
x
1
R₂ x R₅ x C₂ x C₅
(3)
The op amp’s gain is set by the equation:
GAIN = -
R₂
R₁
(4)
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