Building a Simple SPICE
Model for the THS3001
Application
Report
April 1999
Mixed Signal Products
SLOA018
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Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Measuring Zt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3 Measuring Zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Measuring Re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Constructing the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
List of Figures
1 Basic CF Op Amp Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 THS3001 Open Loop Transimpedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Measuring Zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 ZoCL vs Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Measuring Re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Model for zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Simple THS3001 SPICE Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 Comparison of Data Sheet and SPICE Model with Gain = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 Comparison of Data Sheet and SPICE Model with Gain = 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11 Comparison of Data Sheet and SPICE Model with Gain = 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Building a Simple SPICE Model for the THS3001
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SLOA018
Building a Simple SPICE Model for the THS3001
James Karki
ABSTRACT
The application report Voltage Feedback Versus Current Feedback Op Amps SLVA051
outlines the basic operation of a current feedback operational amplifier (op amp) in
relation to a voltage feedback op amp. One of the basic principles of a current feedback
op amp is that its transfer function is a transimpedance equal to the output voltage divided
Vo .
by the current from the negative input terminal, i.e., Zt
le
This application report describes how to construct a simple model that will simulate the
frequency response of the THS3001 in SPICE. The laboratory setup to measure the basic
parameters of the THS3001 is illustrated.
+
1 Introduction
This report highlights the basic functioning of the THS3001 op amp, and helps
give insight to its proper use. Limitations of the device such as input common
mode range, power supply range, CMRR, PSRR, output voltage swing, etc., are
purposely left out . The model is simple and easy to understand, simulates fast,
and gives good results.
Figure 1 is a basic current feedback op amp model. To transform this into a SPICE
circuit model, Zt, Zo, and Re must be determined.
Vp
+
x1
x1
Ie
Vn
Re
Ie
Zo
VO
Zt
–
Figure 1. Basic CF Op Amp Model
2 Measuring Zt
A network analyzer is designed to measure the transfer function of a circuit.
Therefore an HP8753E network analyzer is used to measure the transfer function
of the THS3001. It has a lower frequency limit of 30 kHz. Below 30 kHz a
servo-loop is used. Figure 2 shows the laboratory test setups. The servo-loop is
on the left and the network analyzer is on the right.
1
Measuring Zt
HP 8753E
Network Analyzer
V1
50 Ω
THS3001
+
V1
_
1 kΩ
50 Ω
1 kΩ
+
+
_
THS3001
V2
1 kΩ
10 kΩ
V3
100 pF
V
In
10 kΩ
10 Ω
R2
100 kΩ
Ie
Out
TLE2227
+
V2
_
Ie
10 Ω
(b) Network Analyzer, f ≥ 30 kHz
(a) Servo-Loop, f < 30 kHz
Figure 2. Test Setup
Solving the servo-loop circuit shown in Figure 2 (a), the transimpedance of the
THS3001 is Zt
+ – DDVV12
× R2 . The 100-pF capacitor limits the bandwidth to
159 kHz and the 1 kΩ provides a load.
Using the network analyzer as shown in Figure 2 (b), the transfer functions V2
V1
V
3
and
are measured by switching the input between V2 and V3. To compute
V1
V2 , divide V2 by V3 to get V2 , then multiply by 10 Ω to get the
Zt
le
V1
V1
V3
+
transimpedance of the THS3001. By default the network analyzer displays the
results in dB. Therefore, the mechanics of the transimpedance calculation
are: Zt ( dB )
+ VV21 (dB) * VV31 (dB) ) 20 dB.
Figure 3 is a plot of the THS3001 transimpedance magnitude and phase versus
frequency from data collected using the test setups and methods described
above. The simulation result of the SPICE model, constructed later, is also
plotted.
2
SLOA018
Measuring Zo
MAGNITUDE
140
130
120
SPICE Data
110
Lab Data
dB – Ω
100
90
80
70
60
50
40
30
1.E+02 1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08 1.E+09
1.E+07
1.E+08 1.E+09
f – Frequency
PHASE
20
0
–20
SPICE Data
–40
–60
Lab Data
Deg
–80
–100
–120
–140
–160
–180
–200
–220
–240
1.E+02 1.E+03
1.E+04
1.E+05
1.E+06
f – Frequency – Hz
Figure 3. THS3001 Open Loop Transimpedance
Analyzing this information:
dc gain = 138.5 dB Ω = 8.5 MΩ
dominant pole near 10 kHz,
multiple upper frequency poles above 200 MHz
3 Measuring Zo
With the circuit shown in Figure 4, the amplifier tries to maintain 0 V at its output
terminal. Because of the finite output impedance zo, there is a finite voltage at V2.
By varying the voltage V1 and recording V2, the output impedance, ZoCL , is
calculated by the formula shown.
Building a Simple SPICE Model for the THS3001
3
Measuring Zo
THS3001
V2
+
IeZt
50 Ω
ZO
100 Ω
V1
–
1 kΩ
Zo
CL
+ 100 ×
ȡȧ ȣȧ
Ȣ Ȥ
1
V1 –1
V2
1 kΩ
Figure 4. Measuring Zo
In the closed loop situation shown, Zo CL
transmission as follows:
Zo
+ zo ×
CL
ȡȧ ȣȧ
Ȣ) Ȥ
0 zo, but it is related by the loop
1
1
Zt
1k
At low frequencies ZoCL is very low because Zt is very high, and decreases as
frequency increases. The graph shown in Figure 5 shows the measured ZoCL
from 100 kHz to 1 GHz. The SPICE simulation of the output impedance model
(developed below) configured the same as the test circuit in Figure 4 is also
plotted.
Analyzing the graph in Figure 5: ZoCL increases at approximately 20 dB/dec
from 100 kHz to over 100 MHz; it peaks at 600 MHz, and falls at about 20 dB/dec
to 1 GHz. Correlating ZoCL to Zt, and the above equation: zo appears resistive
up to about 100 MHz where Zt = 1000. After that zo appears reactive in nature.
4
SLOA018
Measuring Re
CLOSED LOOP OUTPUT IMPEDANCE
vs
FREQUENCY
100
ZoCL – Output Impedance – Ω
10
SPICE Data
1
Lab Data
0.1
0.01
0.001
1E+05
1E+06
1E+07
1E+08
1E+09
f – frequency – Hz
Figure 5. ZoCL vs Frequency
4 Measuring Re
By measuring the transfer function V2 in Figure 6, Re is approximated by the
V1
formula shown. At higher frequencies, parasitic inductance and capacitance
modify the impedance, but for the most part, these effects are included in the
measurement of Zt. So, for the purposes of this report, take Re = 25 Ω resistive.
V1
+
_
THS3001
VO
Re
V2
+ 10 ×
ǒ Ǔ+
V1 –1
V2
25 W
10 Ω
Figure 6. Measuring Re
5 Constructing the Model
To build a SPICE model for the THS3001, use Figure 1 as a template, and use
the data collected to compute component values. E and F devices are used to
construct the SPICE model. An E device is a voltage-controlled voltage source
and an F device is a current-controlled current source. Unity gain is used for these
devices.
The input is modeled by an E device driving an F device. The current in the F
device is set by Re between the negative input terminal and ground. The output
of the F device emulates the error current le.
Building a Simple SPICE Model for the THS3001
5
Constructing the Model
The output of the F device drives Rt = 8.5 MΩ and Cc = 1.25 pF. The values are
chosen to set the dc gain and dominant pole of Zt.
Trial and error reveals that an RC single pole at 1 GHz, and an LRC double pole
at 800 MHz (Q = 1) shapes the upper frequency response close to the measured
values. E1 – E3 provide buffering. R1 and C1 form the RC single pole, and L2,
R2, and C2 form the LRC double pole.
The model shown in Figure 7 is proposed for zo.
Rc
C
RI
L
Figure 7. Model for zo
Ǹ
Select component values as follows:
1. Rl = ZoCL divided by Zt/1000 at 100 kHz → Rl ≈ 7.7 Ω
2. sL = Rl at 100 MHz → L ≈ 11 nH
3. 1/sC = sL at 600 MHz → C ≈ 6.3 pF
4. Rc
+
ǒ
Ǔ ǒǓ
RI
* CL ) sC
RI * Zo
) (sL )
CL
2
Zo
ǒ
RI
CL
Ǔ
2
2
, with s
2
+ 2p * 600 MHz, ³ Rc [ 18 W
Figure 8 shows the SPICE model created. The simulated open loop
transimpedance and closed loop output impedance are included in Figure 3 and
Figure 5 to see the correlation between simulating this model and the lab data
collected on the THS3001.
Input Buffer
Ein
+ IN
Dominant Pole
Multiple Upper Frequency Poles
F1
+
+
– IN
Output Impedance
R1
1 kΩ
E1
+
Re
25 Ω
Rt
8.5 MΩ
Cc
1.25 pF
+
E2
+
C1
0.159 pF
L2
+
R2
50 Ω
10 nH
C2
4 pF
Ro
18 Ω
E3
+
+
RI
7.7 Ω
To test the model further, SPICE simulations of circuits from the THS3001 data
sheet, literature number SLOS217, is performed and compared to the data sheet
graphs. The circuits and the results are shown in Figure 9 through Figure 11.
Note that 1-pF capacitors are added to the output and negative input to simulate
parasitic board capacitance. The data from the data sheet is prefaced by DS,
followed by the circuit configuration. The data from the SPICE simulation is
prefaced by SPICE, followed by the circuit configuration.
SLOA018
L2
11 nH
Figure 8. Simple THS3001 SPICE Model
6
C
6.3 pF
OUT
Constructing the Model
GAIN
vs
FREQUENCY
3
(2)
2
(4)
Gain = 1
1 pF†
1
THS3001
Gain – dB
+
_
VI
(1)
VO
1 pF†
Rf
(6)
0
(3)
(1) DS RF = 750 Ω
(2) SPICE RF = 750 Ω
(3) DS RF = 1 kΩ
(4) SPICE RF = 1 kΩ
(5) DS RF = 1.5 kΩ
(6) SPICE RF = 1.5 kΩ
–1
† For SPICE simulation
–2
–3
1.00E+06
(5)
1.00E+07
1.00E+08
1.00E+09
f – Frequency – Hz
Figure 9. Comparison of Data Sheet and SPICE Model with Gain = 1
GAIN
vs
FREQUENCY
9
(1)
8
Gain = 2
1 pF†
Rg
† For SPICE simulation
+
_
7
(3)
THS3001
VO
1 pF†
Rf
6
Gain – dB
VI
(2)
(4)
(6)
5
(5)
4
3
2
1
(1) DS RF = 560 Ω
(2) SPICE RF = 560 Ω
(3) DS RF = 680 Ω
(4) SPICE RF = 680 Ω
(5) DS RF = 1 kΩ
(6) SPICE RF = 1 kΩ
0
1.00E+06
1.00E+07
1.00E+08
f – Frequency – Hz
1.00E+09
Figure 10. Comparison of Data Sheet and SPICE Model with Gain = 2
Building a Simple SPICE Model for the THS3001
7
Summary
GAIN
vs
FREQUENCY
18
17
Gain = 5
1 pF†
Rg
Rf
(2)
15
THS3001
VO
1 pF†
14
Gain – dB
+
_
VI
(1)
16
13
(4)
(5)
12
(6)
11
10
† For SPICE simulation
9
8
7
6
(1) DS RF = 390 Ω
(2) SPICE RF = 390 Ω
(3) DS RF = 560 Ω
(4) SPICE RF = 560 Ω
(5) DS RF = 1 kΩ
(6) SPICE RF = 1 kΩ
5
1.00E+06
1.00E+07
(3)
1.00E+08
1.00E+09
f – Frequency – Hz
Figure 11. Comparison of Data Sheet and SPICE Model with Gain = 5
6 Summary
The simple model developed here is useful for simulating and understanding the
basic operation of the THS3001. Hopefully this model will prove practical to you.
Use it in good health.
8
SLOA018