ISL55211 Datasheet, PDF (15/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain, Differential Amplifier

ISL55211
TABLE 4. OUTPUT NOISE AND INPUT REFERRED EQUIVALENT NOISE FOR THE TRANSFORMER COUPLED INPUT
ISL55211 INTENDED TRANSFORMER + INTERNAL GAIN SETTINGS
INPUT REFERRED
INPUT XFMR
TURNS RATIO
1:1.4
INTERNAL
R_G VALUE (Ω)
250
GAIN (V/V)
V_O/V_I
2.8
GAIN (dB)
V_O/V_I
9
EXTERNAL
R_T VALUE (Ω)
122
TOTAL GAIN
RESISTOR FOR
NG (Ω)
277.48
NOISE GAIN
V/V
2.80
E_O
nV/√Hz
7.94
E_NI
nV/√Hz
2.834811
1:1.4
125
5.6
15
162
155.92
4.21
9.62
1.718338
1:1.4
100
7
17
192
132.88
4.76
10.25
1.46452
1:2
250
4
12
333
312.48
2.60
7.68
1.920066
1:2
125
8
18
1020
208.61
3.40
1:2
100
10
20
100⁸
200.00
3.50
8.67
8.79
1.083876
0.879492
Adding an input transformer can improve the input referred noise
by adding a noiseless voltage gain. Starting from Test Circuit 1 of
Figure 29, and assuming the source shows a matched
broadband source R_S that will be matched by the input referred
parallel combination of 2*R_G||R_T, a noise gain analysis circuit
can be developed as shown in Figure 35.
RF
RT/2
RG
500
+
n²RS/2
ISL55211
n²RS/2
-
RG
RT/2
500
RF
FIGURE 35. NOISE GAIN MODEL FOR THE TRANSFORMER
COUPLED INPUT CIRCUIT OF FIGURE 29
Stepping through the 3 gain settings with two input transformers
will allow the noise gain to be calculated for the circuit of
Figure 35, which is all that is needed in Equation 1 to arrive at an
output differential noise (since R_F is fixed at 500Ω). Doing this
gives Table 4.
The signal gain is taken from the input of the transformer for this
analysis and shows the total input referred noise going below
0.9nV at the highest gain setting here. While this analysis is
including the approximate 0.9nV noise of a 50Ω source R, that
noise is assumed to be divided down by 2 to the input of the
transformer, which explains the total input referred noise
showing up as less than just a 50Ω resistor. The total output
differential noise goes below 9nV/√Hz at the higher gains
settings using this input transformer technique. For even lower
noise, consider the ISL55210 where the input R_T element is
generally not required. In that case, simply setting R_G to the
desired input Z and adjusting R_F to the desired gain will give an
output noise that is slightly lower than shown previously for the
same input transformer due to the removal of the R_T element.
Driving Cap and Filter Loads
Most applications will drive a resistive or filter load. The
ISL55211 is robust to direct capacitive load on the outputs up to
approximately 10pF. For frequency response flatness, it is best to
avoid any output pin capacitance as much as possible - as the
capacitance increases, the high frequency portion of the
ISL55211 (>1GHz) response will start to show considerable
peaking. No oscillations were observed up through 10pF load on
each output.
For AC-coupled applications, an output network that is a small
series resistor (10 to 50Ω) into a blocking capacitor is preferred.
This series resistor will isolate parasitic capacitance to ground
from the internally closed loop output stage of the amplifier and
de-que the self resonance of the blocking capacitors. Once the
output stage sees this resistive element first, the remaining part
of a passive filter design can be done without fear of amplifier
instability.
Driving ADC's
Many of the intended applications for the ISL55211 are as a low
power, very high dynamic range, last stage interface to high
performance ADC's. The lowest power ADC's, such as the
ISLA214P50 shown on the front page, include an innovative
"Femto-Charge™" internal architecture that eliminates op amps
from the ADC design and only passes signal charge from stage to
stage. This greatly reduces the required quiescent power for
these ADC's but then that signal charge has to be provided by the
external circuit at the two input pins. This appears on an ADC like
the ISLA112P50 as a clock rate dependent common mode input
current that must be supplied by the interface circuit. At
500MHz, this DC current is 1.3mA on each input for the 14-bit
ISLA214P50.
Most interfaces will also include an interstage noise power
bandlimiting filter between the amplifier and the ADC. This filter
needs to be designed considering the loading of the amplifier,
any V_CM level shifting that needs to take place, the filter shape,
and this Icm issue into the ADC input pins. Here are 4 example
topologies suitable for different situations.
1. AC-coupled, broadband RLC interstage filter design. This
approach lets the amplifier operate at its desired output
common mode, then provides the ADC common mode
voltage and current through a bias path as part of the filters
designs last stage R values. The V_b is set to include the IR loss
from that voltage to the ADC inputs due to the I_CM current.
15
FN7868.0
June 21, 2011