Waterloo Instruments
EM-16
16-Channel EMG Amplifier
User’s
Manual
[RH1]
Package Contents
Your package should include:
· EM-16 Amplifier
· BNC Output Module
· ±12V DC Power Supply
· 50-pin Output Cable
· Input Connectors
Figure 1 : EM-16 Package
Overview
The EM-16 provides 16 channels of amplification and filtering, with the following specifications:
· Selectable voltage gain: 10, 100 or 1000.
· Differential inputs.
· 22MW input impedance.
· 93dB to 130dB common-mode rejection[RH2].
· Configurable one-pole low-pass and high-pass filters.
In addition, the EM-16 includes the following features:
· Built-in amplified speaker, to monitor selected channels.
· Built-in calibration pulse generator (50 or 100mV pulses).
All 16 inputs are on a single DB-37F connector, on the top edge of the EM-16 chassis. The pinout of this connector is described below. All 16 outputs are on a single 50-pin Centronics-type connector. A single 50-conductor cable will carry all outputs to a separate output box, located outside the Faraday cage. The output box breaks the 16 outputs onto separate BNC-type connectors.
Differential
Amplification
The inputs to the EM-16 are configured as differential inputs. Each channel input actually consists of two inputs, called the non-inverting and inverting inputs. The inputs are often labelled ‘+’ and ‘-‘, respectively. The amplifier amplifies the difference between the two inputs, multiplying the difference by the selected gain (10, 100 or 1000). Any signal that is common to both inputs (called a common-mode signal) is ignored.
Neither of the two inputs is required to be connected to a common ground, but either may be.
By comparison, a single-ended amplifier has a single input, and amplifies the difference between that input and a common ground reference.
The advantage of a differential input is noise-immunity. A signal driven by a high-impedance source (such as an EMG, EKG or ECG probe, or intracellular recording probe) is highly susceptible to noise and hum picked up from the surrounding environment. But if that signal is carried differentially, then noise-immunity is greatly increased. The reason is that any noise or hum picked up by the differential signal is likely to be picked up equally on both inverting and non-inverting inputs (because the two signal conductors are usually run in parallel, very close together). The noise is thus largely a common-mode signal, and will not be amplified.
]In practice, no amplifier is perfect. Common-mode input signals are actually passed through the amplifier slightly.
Installation and Setup
Specimen Connection
The signal sources must be connect
Changing the Filter Settings
The EM-16 is shipped with the filters set for a 30Hz high-pass and 50kHz low-pass. The filter settings can be changed, but some effort is required. The cut-off frequencies are determined by capacitors installed inside the EM-16. The capacitors are connected by screw-terminals, so changing them is not difficult. But it does require opening the chassis.
As always when working with electronics, it pays to take precautions against static electricity, which can damage sensitive components. Ideally, you should wear a specially made static-dissipative wrist-strap connected to a good ground point, and do your work on a static-dissipative surface.
Here are the steps required to disassemble the unit:
1. Disconnect the power, input and output cables.
2. Place the unit face-down on a soft, preferably static-dissipative surface. Remove the eight screws on the edges of the chassis.
3. Carefully lift the rear cover off the unit a short distance. There are four cables connecting the rear of the chassis to the circuit board. Reach in with your hand, and carefully disconnect them. Make note of where each cable was connected. Disconnect the two ribbon cables first. Then, the back cover can be hinged on one end allowing access to the power and speaker cables. Disconnect them. Set the rear cover aside.
For each channel, you will see a black 3-pin screw-terminal block, as illustrated in Figure 2. The cut-off frequencies are set by capacitors installed in the black terminal blocks. Table 1and Table 2 show the capacitance values required for various cut-off frequencies. The capacitors used must be bipolar, and rated for at least ±20V. Ceramic or plastic types are recommended. Polarized capacitor types such as electrolytic or tantalum must not be used.
For some capacitors, the leads are too thin to make reliable contact with these terminals. It helps to bend the leads double before inserting them into the terminal.
Figure 2: Filter Capacitors
|
Highpass (Hz) |
Capacitance |
Code |
Provided |
|
0.1 Hz |
1.64 mF |
|
|
|
0.3 Hz |
0.27 mF |
|
|
|
1 Hz |
0.082 mF |
|
|
|
3 Hz |
0.027 mF |
273 |
ü |
|
10 Hz |
0.0082 mF |
8n2 or 822 |
ü |
|
30 Hz |
0.0027 mF |
2n7 or 272 |
ü (default) |
|
100 Hz |
820 pF |
821 |
ü |
|
300 Hz |
260 pF |
|
|
Table 1 : High-pass Filter Settings
|
Lowpass (Hz) |
Capacitance |
Provided |
|
10 Hz |
0.82 mF |
|
|
30 Hz |
0.27 mF |
|
|
100 Hz |
0.082 mF |
|
|
300 Hz |
0.027 mF |
ü |
|
1 kHz |
0.0082 mF |
ü |
|
3 kHz |
0.0027 mF |
ü |
|
10 kHz |
820 pF |
ü |
|
50 kHz |
None |
|
Table 2 : Low-pass Filter Settings
Reassemble the unit by reversing the steps for disassembly.
Input Connector Pinouts
The input connector is a 37-pin female DB-type connector. Figure 3 shows the pinout, as viewed from the top of the EM-16 chassis. The pin numbers are also printed (in tiny digits) on the connector itself.
Figure 3: Input connector
|
Pin Number |
Description |
Pin Number |
Description |
|
1 |
Chan 1 + |
20 |
Chan 1 - |
|
2 |
Chan 2 + |
21 |
Chan 2 - |
|
3 |
Chan 3 + |
22 |
Chan 3 - |
|
4 |
Chan 4 + |
23 |
Chan 4 - |
|
5 |
Chan 5 + |
24 |
Chan 5 - |
|
6 |
Chan 6 + |
25 |
Chan 6 - |
|
7 |
Chan 7 + |
26 |
Chan 7 - |
|
8 |
Chan 8 + |
27 |
Chan 8 - |
|
9 |
Chan 9 + |
28 |
Chan 9 - |
|
10 |
Chan 10 + |
29 |
Chan 10 - |
|
11 |
Chan 11 + |
30 |
Chan 11 - |
|
12 |
Chan 12 + |
31 |
Chan 12 - |
|
13 |
Chan 13 + |
32 |
Chan 13 - |
|
14 |
Chan 14 + |
33 |
Chan 14 - |
|
15 |
Chan 15 + |
34 |
Chan 15 - |
|
16 |
Chan 16 + |
35 |
Chan 16 - |
|
17 |
n.c. |
36 |
n.c. |
|
18 |
n.c. |
37 |
n.c. |
|
19 |
n.c. |
|
|
Table 3: Input Connector Pinout
(n.c.: no connection)
The inputs of the EM-16 are “differential” inputs, meaning that the difference between the non-inverting or + input and inverting or “-“ input is amplified. Any signal common to both inputs will be not be amplified.
Output Connector Pinouts
The output connector is on the rear panel. It is a 50-pin female Centronics-type connector. Figure 4 shows the pinout of the output connector, as viewed from the rear of the EM-16. Table 4 describes the function of each pin.
Figure 4: Output Connector
|
Pin Number |
Description |
|
1 |
Output 1 |
|
2 |
Output 2 |
|
3 |
Output 3 |
|
4 |
Output 4 |
|
5 |
Output 5 |
|
6 |
Output 6 |
|
7 |
Output 7 |
|
8 |
Output 8 |
|
9 |
Output 9 |
|
10 |
Output 10 |
|
11 |
Output 11 |
|
12 |
Output 12 |
|
13 |
Output 13 |
|
14 |
Output 14 |
|
15 |
Output 15 |
|
16 |
Output 16 |
|
17-25 |
n.c. |
|
26-41 |
ground |
|
42-50 |
n.c. |
Table 4 : Output Connector Pinout
Theory
of Operation
Figure 5 shows a block-diagram of a single channel of the amplifier.
Figure 5: Single Channel Diagram
Each channel of the EM-16 is based on the AD620 Instrumentation Amplifier, produced by Analog Devices. The data-sheet for that part is included in the appendix. The inputs to the AD620 are selected by a DPDT switch, from either the external differential inputs, or from the internal calibration pulse bus.
The gain of the AD620 is controlled by a resistance between two terminals. In the EM-16, that resistance is determined by a rotary switch, selecting resistances for gains of 10, 100 or 1000. The resistors are 1% tolerance types.
The output of the AD620 is passed through single-pole RC low-pass and high-pass filters. The capacitors in the RC filters are attached to screw terminal blocks, so they can be changed. The output of the RC filters is buffered by a TL071 op-amp, connected as a non-inverting buffer. This buffer stage ensures consistent performance from the RC filters, regardless of load impedance.
The output from the TL071 buffer goes to the rear-panel output connector, and also to a switch which enables the channel output onto the monitor mixing bus.
Calibration Supply
The calibration voltages (50 or 100mV) are derived by a resistive voltage divider from a National Semiconductor LM336 precision 2.5V reference. The LM336 behaves essentially like a zener-diode reference, but with much better accuracy and stability. The LM336 provides a ±1% tolerance on the output voltage. The resistors in the voltage divider are 1% tolerance types.
A 3-position rotary switch selects 0V, or a 50mV or 100mV tap in the voltage divider. In addition, a second pole on the rotary switch supplies power to the LM336 if the 50mV or 100mV position is selected. A momentary switch supplies the 2.5V reference voltage to the voltage divider, to generate the actual 50mV or 100mV pulse.
The calibration pulses are routed to the
input select switches of all channels.
Monitor Amplifier
Each channel has a switch that gates that channel’s output to the monitor amplifier. The monitor amplifier mixes the outputs from all enabled channels, using a simple inverting summing amplifier based on a TL071 op-amp. The output from the mixer (the sum of all enabled channel outputs) is then passed (through a volume control potentiometer) to an LM386 audio power amplifier, and thence to a small internal loudspeaker.
A 1/8” phone jack is provided to allow the use of an external speaker. When an external speaker is connected, the internal speaker will be disconnected automatically. The external speaker can help avoid feedback.
Finally, a ganged switch on the volume control will disconnect power to the LM386 when the volume is turned all the way down.